Gracefully handle invalid resources when updating menu/toolbar status

[jng]

Some commands in Maestro are enabled/disabled based on some state of the active editor and it usually involves its associated resource. If this resource is invalid for any reason that would throw up an exception it will derail Maestro.

As it stands, wrapping a try/catch around the status update and condition evaluation code will not suffice as the current code will simply spam the MapGuide Server for the same invalid resource.

We need a mechanism where if the active editor's resource is invalid that there be some way for the editor to mark such resource as invalid so that the code will know not to spam the MapGuide Server for the same invalid resource.

[jng]

Fixed r7924