NetworkMenu: use network-offline while loading

network-error is partially red and thus can be distracting at
login. Use network-offline instead.

https://bugzilla.gnome.org/show_bug.cgi?id=674426

js/ui/status/network.js

@@ -1538,7 +1538,7 @@ const NMApplet = new Lang.Class({
     Extends: PanelMenu.SystemStatusButton,
 
     _init: function() {
-        this.parent('network-error', _("Network"));
+        this.parent('network-offline', _("Network"));
 
         this._client = NMClient.Client.new();