Conflicts:
mopidy/mpd/frontend.py
tests/mpd/frontend_test.py
GstreamerBackend now raises LookupError where the other backends just
returns None. We'll probably move in the direction of using LookupError
all over, but the changes to DummyBackend and generic tests was reverted
until BaseBackend, the backend API docs and the other backends are
updated to all raise LookupError.