Make Contract messages more user-friendly

[averagehat]

e.g.

contracts.interface.ContractNotRespected: Breach for argument 'outdir' to run_sample_sheet().
Shouldn't have satisfied the clause exists.
checking: !exists for value: Instance of <type 'str'>: './'
checking: str,!exists for value: Instance of <type 'str'>: './'
Variables bound in inner context:

Could become

contracts.interface.ContractNotRespected: Breach for argument 'outdir' to run_sample_sheet().
=====================
ERROR:  blah blah blah
======================
checking: !exists for value: Instance of <type 'str'>: './'
checking: str,!exists for value: Instance of <type 'str'>: './'
Variables bound in inner context:

Do this by raising a ValueError rather than returning false. Example:

def fails(x):
    raise ValueError("="*9 + "\nERROR: blah blah blah, do xyz\n" + "="*9)
contracts.new_contract("failit", fails)
contracts.decorate(lambda x: x, x="failit")(1)

I would just wrap our current boolean-returning functions

def wrap_validation(func, message):
   def ret_func(arg):
      if not func(arg): 
         raise ValueError(message)
   return ret_func

contracts.new_contract("is_truthy", wrap_validation(lambda x: bool(x), "Not truthy, use truthy input"))

[necrolyte2]

It needs to be very explicit in what is wrong, that is, it should say exactly what the issue is

Like for the !exists issue, it sould say something like

Error: Directory X already exists

[averagehat]

This is better as a PR against the contracts library.