eliminate g_error() usage

Replaced all occurrences of g_error() with MPD_ERROR() located in a new header
file 'mpd_error.h'. This macro uses g_critical() to print the error message
and then exits gracefully in contrast to g_error() which would internally call
abort() to produce a core dump.

The macro name is distinctive and allows to find all places with dubious error
handling. The long-term goal is to get rid of MPD_ERROR() altogether. To
facilitate the eventual removal of this macro it was added in a new header
file rather than to an existing header file.

This fixes #2995 and #3007.

src/encoder/vorbis_encoder.c

@@ -22,6 +22,7 @@
 #include "encoder_plugin.h"
 #include "tag.h"
 #include "audio_format.h"
+#include "mpd_error.h"
 
 #include <vorbis/vorbisenc.h>
 
@@ -374,7 +375,7 @@ vorbis_encoder_read(struct encoder *_encoder, void *_dest, size_t length)
 
 	if (nbytes > length)
 		/* XXX better error handling */
-		g_error("buffer too small");
+		MPD_ERROR("buffer too small");
 
 	memcpy(dest, page.header, page.header_len);
 	memcpy(dest + page.header_len, page.body, page.body_len);