crtauth should not raise RuntimeErrors

[negz]

https://github.com/spotify/crtauth/search?q=RuntimeError&type=Code
http://docs.python.org/release/2.6.8/library/exceptions.html#exceptions.RuntimeError

crtauth seems to use RuntimeError for a subset of its exceptions. This makes it difficult for consumer code to isolate crtauth exceptions. crtauth should use its own Exception classes.

It also seems like a misleading error to raise - while the documentation doesn't say explicitly I would expect RuntimeErrors to be raised by the Python runtime for things that didn't fall elsewhere.