Cleanup, tests and docs for Exceptions and Errors

[psarka]

The issue #72 was caused by 0.16.1 raising different exception than 0.16 - TypeError instead of threading.ThreadError. The former does not inherit from Exception, so duplicity failed to intercept it when releasing the lock twice (they always release the lock twice).

We need tests to make sure we maintain the exception interface, as well as docs. And ideally we should not be raising threading.ThreadError, as processes have nothing to do with that.