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/decoder_list.c

@@ -22,6 +22,7 @@
 #include "decoder_plugin.h"
 #include "utils.h"
 #include "conf.h"
+#include "mpd_error.h"
 
 #include <glib.h>
 
@@ -198,8 +199,8 @@ decoder_plugin_config(const char *plugin_name)
 		const char *name =
 			config_get_block_string(param, "plugin", NULL);
 		if (name == NULL)
-			g_error("decoder configuration without 'plugin' name in line %d",
-				param->line);
+			MPD_ERROR("decoder configuration without 'plugin' name in line %d",
+				  param->line);
 
 		if (strcmp(name, plugin_name) == 0)
 			return param;