Fail with a helpful message if separate cache is not supported

[tkf]

It needs some discussion. Also, #183 has to be merged first.

With this PR, user will see error message like this which saves their time for googling the issues.

RuntimeError: It seems your Julia and PyJulia setup are not supported.

Julia interpreter:
    /home/takafumi/bin/julia-1.0.0
Python interpreter used by PyCall.jl:
    /home/takafumi/miniconda3/bin/python
Python interpreter used to import PyJulia.
    /usr/bin/python

In Julia >= 0.7, above two paths to the Python interpreters have to match
exactly in order for PyJulia to work.  To configure PyCall.jl to use Python
interpreter "/usr/bin/python",
run the following commands in the Julia interpreter:

    ENV["PYTHON"] = "/usr/bin/python"
    using Pkg
    Pkg.build("PyCall")

For more information, see:
    https://github.com/JuliaPy/pyjulia
    https://github.com/JuliaPy/PyCall.jl

I'm not 100% sure we want this. We should probably be better just support it. Supporting it via DEPOT_PATH may not be so bad (#173).

ref: #185

[mprogram]

(related to #185):

In the present case, it is a bit confusing as those two values are identical:

# python3
Python 3.6.5 (default, Apr  1 2018, 05:46:30) 
[GCC 7.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import julia; julia.Julia(debug=True)

┌ Warning: `Pkg.dir(pkgname, paths...)` is deprecated; instead, do `import PyCall; joinpath(dirname(pathof(PyCall)), "..", paths...)`.
└ @ Pkg.API /build/julia-gZj_Ru/julia-0.7.0/usr/share/julia/stdlib/v0.7/Pkg/src/API.jl:454
pyprogramname = /usr/bin/python3
sys.executable = /usr/bin/python3
exe_differs = False
JULIA_HOME = /usr/bin,  libjulia_path = /usr/bin/../lib/x86_64-linux-gnu/libjulia.so.0.7
calling jl_init_with_image(b'/usr/lib/python3/dist-packages/julia/fake-julia', /usr/lib/x86_64-linux-gnu/julia/sys.so)
seems to work...
exception occured? None
exception occured? None
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3/dist-packages/julia/core.py", line 646, in __init__
    self.__julia = Julia(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/julia/core.py", line 492, in __init__
    raise_separate_cache_error(runtime, depsjlexe)
  File "/usr/lib/python3/dist-packages/julia/core.py", line 320, in raise_separate_cache_error
    raise RuntimeError(message)
RuntimeError: It seems your Julia and PyJulia setup are not supported.

Julia interpreter:
    julia
Python interpreter used by PyCall.jl:
    /usr/bin/python3
Python interpreter used to import PyJulia.
    /usr/bin/python3

In Julia >= 0.7, above two paths to the Python interpreters have to match
exactly in order for PyJulia to work.  To configure PyCall.jl to use Python
interpreter "/usr/bin/python3",
run the following commands in the Julia interpreter:

    ENV["PYTHON"] = "/usr/bin/python3"
    using Pkg
    Pkg.build("PyCall")

For more information, see:
    https://github.com/JuliaPy/pyjulia
    https://github.com/JuliaPy/PyCall.jl

# ldd $(which python3)
	linux-vdso.so.1 (0x00007ffed6dec000)
	libgtk3-nocsd.so.0 => /usr/lib/x86_64-linux-gnu/libgtk3-nocsd.so.0 (0x00007f03eed27000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f03eeb08000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f03ee904000)
	libutil.so.1 => /lib/x86_64-linux-gnu/libutil.so.1 (0x00007f03ee701000)
	libexpat.so.1 => /lib/x86_64-linux-gnu/libexpat.so.1 (0x00007f03ee4cf000)
	libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007f03ee2b2000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f03edf14000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f03edb23000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f03eef2e000)

[tkf]

Thanks a lot for trying it out! Yes, it has to detect if Python is statically linked or not and then prints the right message.

[stevengj]

Needs a rebase.

[coveralls]

Pull Request Test Coverage Report for Build 478

• 22 of 39 (56.41%) changed or added relevant lines in 1 file are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-1.09%) to 77.341%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
julia/core.py	22	39	56.41%

Files with Coverage Reduction	New Missed Lines	%
julia/core.py	1	81.87%
julia/find_libpython.py	2	79.83%

Totals
Change from base Build 469:	-1.09%
Covered Lines:	413
Relevant Lines:	534

---

💛 - Coveralls

[tkf]

Rebased and added some more tests. I think it's good to go.

For incompatible libpython (but dynamically linked), it now prints:

$ python2 -c "from julia import Main"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "julia/core.py", line 172, in load_module
    JuliaMainModule(self, fullname))
  File "julia/core.py", line 77, in __init__
    self._julia = loader.julia
  File "julia/core.py", line 164, in julia
    self.__class__.julia = julia = Julia()
  File "julia/core.py", line 574, in __init__
    raise_separate_cache_error(runtime, jlinfo)
  File "julia/core.py", line 404, in raise_separate_cache_error
    raise RuntimeError(message)
RuntimeError: It seems your Julia and PyJulia setup are not supported.

Julia interpreter:
    julia
Python interpreter and libpython used by PyCall.jl:
    /home/takafumi/.virtualenvs/julia1.0/bin/python3
    /usr/lib/libpython3.7m.so.1.0
Python interpreter used to import PyJulia and its libpython.
    /usr/bin/python2
    /usr/lib/libpython2.7.so.1.0

In Julia >= 0.7, above two paths to `libpython` have to match exactly
in order for PyJulia to work.  To configure PyCall.jl to use Python
interpreter "/usr/bin/python2",
run the following commands in the Julia interpreter:

    ENV["PYTHON"] = "/usr/bin/python2"
    using Pkg
    Pkg.build("PyCall")

For more information, see:
    https://github.com/JuliaPy/pyjulia
    https://github.com/JuliaPy/PyCall.jl

When libpython is statically linked:

$ ~/miniconda3/bin/python -c "from julia import Main"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "<frozen importlib._bootstrap>", line 971, in _find_and_load
  File "<frozen importlib._bootstrap>", line 955, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 656, in _load_unlocked
  File "<frozen importlib._bootstrap>", line 626, in _load_backward_compatible
  File "/home/takafumi/repos/watch/pyjulia/julia/core.py", line 172, in load_module
    JuliaMainModule(self, fullname))
  File "/home/takafumi/repos/watch/pyjulia/julia/core.py", line 77, in __init__
    self._julia = loader.julia
  File "/home/takafumi/repos/watch/pyjulia/julia/core.py", line 164, in julia
    self.__class__.julia = julia = Julia()
  File "/home/takafumi/repos/watch/pyjulia/julia/core.py", line 574, in __init__
    raise_separate_cache_error(runtime, jlinfo)
  File "/home/takafumi/repos/watch/pyjulia/julia/core.py", line 404, in raise_separate_cache_error
    raise RuntimeError(message)
RuntimeError: It seems your Julia and PyJulia setup are not supported.

Julia interpreter:
    julia
Python interpreter and libpython used by PyCall.jl:
    /home/takafumi/.virtualenvs/julia1.0/bin/python3
    /usr/lib/libpython3.7m.so.1.0
Python interpreter used to import PyJulia and its libpython.
    /home/takafumi/miniconda3/bin/python
    /home/takafumi/miniconda3/lib/libpython3.6m.so.1.0

Your Python interpreter "/home/takafumi/miniconda3/bin/python"
is statically linked to libpython.  Currently, PyJulia does not support
such Python interpreter.  For available workarounds, see:
    https://github.com/JuliaPy/pyjulia/issues/185

For more information, see:
    https://github.com/JuliaPy/pyjulia
    https://github.com/JuliaPy/PyCall.jl