pithos: Backend refactoring

Remove 'pithos.backends.base' module:
* Move backend method docstrings to ModularBackend class.
* Define backend exceptions in a separate module.
pithos: Provide descriptions for backend errors

Closes #1
pithos: Handle backend errors centrally
pithos: Cleanup 'pithos.api.util'

Import code's readability
pithos: PEP8 fixes