Make plugin loading failure fatal

If a plugin was explicitly specified, falling back to the default
silently is bad; a manager component like gnome-session can
do a better job of handling this scenario.

An example we've hit in gnome-shell is where SpiderMonkey changes
without gjs being rebuilt, and loading the plugin fails due to
unresolved symbols.

But there are obviously others, like the file being missing or
corrupt.

https://bugzilla.gnome.org/show_bug.cgi?id=641384

src/compositor/meta-plugin-manager.c

@@ -31,6 +31,7 @@
 #include "../core/window-private.h"
 
 #include <string.h>
+#include <stdlib.h>
 
 #include <clutter/x11/clutter-x11.h>
 
@@ -281,8 +282,15 @@ meta_plugin_manager_load (MetaPluginManager *plugin_mgr)
                 }
             }
           else
-            g_warning ("Unable to load plugin module [%s]: %s",
+            {
+              /* This is fatal under the assumption that a monitoring
+               * process like gnome-session will take over and handle
+               * our untimely exit.
+               */
+              g_printerr ("Unable to load plugin module [%s]: %s",
                           path, g_module_error());
+              exit (1);
+            }
 
           g_free (path);
           g_free (plugin_string);