docs: Add {Backend,Frontend,Mixer}Error guidelines to backend/frontend/mixer APIs

(cherry picked from commit 83f1d00944)

Conflicts:
	mopidy/mixer.py

docs/api/frontends.rst

@@ -29,12 +29,12 @@ The following requirements applies to any frontend implementation:
 - The main actor MUST be able to start and stop the frontend when the main
   actor is started and stopped.
 
-- The frontend MAY require additional settings to be set for it to
-  work.
+- The frontend MAY require additional config values to be set for it to work.
 
-- Such settings MUST be documented.
+- Such config values MUST be documented.
 
-- The main actor MUST stop itself if the defined settings are not adequate for
+- The main actor MUST raise the :exc:`~mopidy.exceptions.FrontendError` with a
+  descriptive error message if the defined config values are not adequate for
   the frontend to work properly.
 
 - Any actor which is part of the frontend MAY implement the

mopidy/backend/__init__.py

@@ -6,6 +6,14 @@ from mopidy import listener
 
 
 class Backend(object):
+    """Backend API
+
+    If the backend has problems during initialization it should raise
+    :exc:`~mopidy.exceptions.BackendError` with a descriptive error message.
+    This will make Mopidy print the error message and exit so that the user can
+    fix the issue.
+    """
+
     #: Actor proxy to an instance of :class:`mopidy.audio.Audio`.
     #:
     #: Should be passed to the backend constructor as the kwarg ``audio``,