Ampache: Handle exception cases of `get_art` consistently
Ampache: Handle exception cases of `get_art` consistently

Previously, calling the action `get_art` for an entity having no art
was responded with HTTP error 404 while calling it for an
invalid/unknown entity was responded with HTTP 200 and XML error
message. Now both of these cases return HTTP 404 which matches the
Ampache API 5 specification. (The API 4 specification doesn't describe
the expected output, but Ampache 4.x seems to return HTTP 200 with mime
text/xml and empty content).

paulijar/music,owncloud/music,paulijar/music,paulijar/music,paulijar/music,paulijar/music,owncloud/music,owncloud/music,owncloud/music,owncloud/music