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/output/shout_plugin.c

@@ -21,6 +21,7 @@
 #include "output_api.h"
 #include "encoder_plugin.h"
 #include "encoder_list.h"
+#include "mpd_error.h"
 
 #include <shout/shout.h>
 #include <glib.h>
@@ -101,8 +102,8 @@ static void free_shout_data(struct shout_data *sd)
 #define check_block_param(name) {		  \
 		block_param = config_get_block_param(param, name);	\
 		if (!block_param) {					\
-			g_error("no \"%s\" defined for shout device defined at line " \
-				"%i\n", name, param->line);		\
+			MPD_ERROR("no \"%s\" defined for shout device defined at line " \
+				  "%i\n", name, param->line);		\
 		}							\
 	}