Don't need to clear the caller-specified one.

playlist_domain is no longer used for class Error.

Improved exception-safety.

See http://bugs.musicpd.org/view.php?id=4496

See http://bugs.musicpd.org/view.php?id=4485

MPD does not really take advantage of memory-mapped I/O by generating
data right into the ALSA buffer; using plain snd_pcm_mmap_writei() has
no advantage compared to snd_pcm_writei().  Let's kill this
non-feature.

Fixes the --disable-dsd build.

See http://bugs.musicpd.org/view.php?id=4498

Fix regression by commit 45f6129a

See http://bugs.musicpd.org/view.php?id=4477

Keep only the last error.  This fixes bogus aborts due to the error
check in decoder_get_virtual_command().

Necessary to preserve type information.  The try/catch sequence didn't
work previously.

Same fix as in commit 1c904000