The _load_library method on BasePasswordHasher turns ImportErrors
into ValueErrors, this masks ImportErrors in the algorithm library.
Changed it to a clearer worded error message that includes
the ImportError string.
* py-bcrypt has not been updated in some time
* py-bcrypt does not support Python3
* py3k-bcrypt, a port of py-bcrypt to python3 is not compatible
with Django
* bcrypt is supported on all versions of Python that Django
supports