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

js/ui/sessionMode.js

@@ -13,6 +13,11 @@ const _modes = {
     'user': { sessionType: Shell.SessionType.USER }
 };
 
+function modeExists(mode) {
+    let modes = Object.getOwnPropertyNames(_modes);
+    return modes.indexOf(mode) != -1;
+}
+
 const SessionMode = new Lang.Class({
     Name: 'SessionMode',