plugin/auditory.vim: Show warning if mplayer not installed

On plugin load, if mplayer isn't installed then fail ungracefully. Since
we depend on mplayer in order to play any sounds, my thinking now is
that I want to fail immediately.

Still trying to decide if this is be best course of action. Maybe I just
want an informative message instead of a complete failure.
plugin/auditory.vim: Decrease warning level of mplayer error

Instead of echoing and error, just echo a message about mplayer not
being installed. Errors look pretty annoying on initial load, and I
don't think this is worth the red background colour. It could be that a
user has two machines, one with mplayer installed and one without. In
that case, I don't want people to have to disable the plugin on one
machine. That seems like too much to ask.

I think this is a nice middle-ground between alerting users that
something is needed by the plugin and not barfing if it's not there.
Update TODO

Check off adding an error message when mplayer is missing.
Merge branch 'error-message-if-no-mplayer'