Improve playlist_open().

Improve playlist_open().

It's easier to let playlist_open() fill in the error text than
to do this in the caller. This is also how the counterpart of
playlist_open(), change_current_mood(), is implemented. Drop the
server log message because this error is usually caused by a client
passing a misspelt playlist name.

Fix the documentation of the return value of the function
while at it: It returns the playlist length on success, and
activate_mood_or_playlist(), its single caller in afs.c, depends
on that.