Fix incorrect application state after a failed attempt to load a PKCS#11 library

[fuzzykat]

Calling the method self.pkcs11.load(lib) twice leads to an incorrect state of the application, if the first attempt to load a library failed. The problem is that code in __init__.py fills the map _loaded_libs with the library name before the library is actually loaded, and then the next call to self.pkcs11.load(lib) finds the cached library (which wasn't properly loaded) and finishes successfully.
The bug can be reproduced by simply setting an invalid PKCS#11 library name and executing the unit tests:

export PYKCS11LIB=/path/to/invalid/lib.so
./run_test.py

which in my case ended up with a segfault.
This PR proposes a fix for the problem.

[coveralls]

coverage: 88.33% (+0.02%) from 88.308%
when pulling 1287b8a on fuzzykat:bugfix/cached_libname
into eaded3b on LudovicRousseau:master.

[LudovicRousseau]

Thanks