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/sig_handlers.c

@@ -25,6 +25,7 @@
 #include "log.h"
 #include "main.h"
 #include "event_pipe.h"
+#include "mpd_error.h"
 
 #include <glib.h>
 
@@ -46,7 +47,7 @@ static void
 x_sigaction(int signum, const struct sigaction *act)
 {
 	if (sigaction(signum, act, NULL) < 0)
-		g_error("sigaction() failed: %s", strerror(errno));
+		MPD_ERROR("sigaction() failed: %s", strerror(errno));
 }
 
 static void