exc: Test ScannerError

(cherry picked from commit 8f59dd69adca2e6c1595159e5350bf5a4e69988f)
exc: Add {Backend,Frontend,Mixer}Error exceptions

(cherry picked from commit bf8307f329ce03b7a311d62aa50fdd7833048f32)
main: Log and exit if {Backend,Frontend,Mixer}Error is raised

(cherry picked from commit 95bddf666bedf1c8fd644042ae6fbf8adca43027)

Conflicts:
	mopidy/commands.py
docs: Add {Backend,Frontend,Mixer}Error guidelines to backend/frontend/mixer APIs

(cherry picked from commit 83f1d00944eaa2d22bffd9fc35ee3a9d990dfe41)

Conflicts:
	mopidy/mixer.py
mpd: Raise FrontendError instead of sys.exit(1)