For the default: case, just use the error message that libFLAC
provides instead of using something ambiguous.  Also, this gets
rid of long lines in the code, making it easier to digest.

Of course, we save ~100 bytes of text space in the process :)

git-svn-id: https://svn.musicpd.org/mpd/trunk@6830 09075e82-0dd4-0310-85a5-a0d7c8717e4f