main: Make sure that --mode parameter is valid

Instead of falling back to a set of default values or crashing the
window manager when an invalid mode is specified, check the value
of the ShellGlobal:session-mode property before taking over as WM
and make a clean exit if it cannot be resolved to an existent mode.

https://bugzilla.gnome.org/show_bug.cgi?id=676156

src/main.c

@@ -335,6 +335,12 @@ main (int argc, char **argv)
 
   _shell_global_init ("session-mode", session_mode, NULL);
 
+  if (!_shell_global_has_valid_session_mode (shell_global_get ()))
+    {
+      g_printerr ("\"%s\" is not a valid mode.\n", session_mode);
+      exit(1);
+    }
+
   ecode = meta_run ();
 
   if (g_getenv ("GNOME_SHELL_ENABLE_CLEANUP"))