Set a JACK error message handler when initializing the backend

Set a JACK error message handler when initializing the backend

JACK2 will print error messages to stderr if it fails to connect to a server.
Users who don't normally use JACK but have the client lib installed will get
those messages even though OpenAL Soft will continue on to find a working
backend without trouble. So to avoid it, set an error message handler that'll
log them as warnings.

This isn't that great because there's no way to tell whether the error messages
are due to the server not running, or some other problem. And it resets the
callback to the default afterward even if it may have been set to something
else before. JACK2, which is what needs this workaround in the first place,
doesn't export the jack_error_callback pointer to properly save and restore it.