app: cleanly remove log handlers on exit

Remove the log handlers registered in errors_init(), in
errors_exit(), and call errors_exit() before destroying the Gimp
instance, since the log handlers depend on it.  This avoids
segfaulting if a message is logged after destroying the Gimp
instance.

app/app.c

@@ -441,11 +441,12 @@ app_run (const gchar         *full_prog_name,
 
   gimp_gegl_exit (gimp);
 
+  errors_exit ();
+
   g_object_unref (gimp);
 
   gimp_debug_instances ();
 
-  errors_exit ();
   gegl_exit ();
 }