Fix #867: Added a SUBSONIC_DEFAULT_TRANSCODING_FORMAT env var to support clients that don't provide the format parameter

The getSong API endpoint is used by, e.g., the subsonic plugin to mopidy.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>

A failed cast to int will raise ValueError, which is not currently caught
by the error checking code, leading to a crash. Fix this so a proper error
message can be returned.

Also add test for getting artist with non-numeric ID.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>