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

Since commit 122527366668825f4c9e63de9f3a2236d94006f9
the backend provides meaningful descriptions for backend exceptions.
This facilitates the frontend to handle centrally such exceptions.
pithos: Cleanup 'pithos.api.util'

Improve code's readability
pithos: PEP8 fixes