Change some exceptions to return values

[flacjacket]

Modify Backend.start and Output.commit to return boolean values rather than checking the return value and raising an error. Move the logic for the Pythonic APIs into the Python-specific functions, in particular the __enter__ and __exit__ so these objects will be well behaved when using the context manager.

Closes #135
Closes #136

[flacjacket]

@heuer Let me know if you think this would address your tickets!

[heuer]

Thanks, looks good to me.

I am not sure if if not self.test(): self.rollback() is necessary (line 163), though.
The wlr-docs mention in the description of the wlr_output_commit function:

[…] On failure, the pending changes are rolled back. […]

So wlroots should do the right thing (rollback) without the check?