Use a more clearer error message for session keep alive if ping fails

[jng]

Maestro uses a keep-alive timer to periodically ping the mapguide server to keep the session alive. Should the mapguide server go offline, you get a full ugly MgConnectionFailedException message.

We know what this is, so this error should be cleaned up to be something more clearer like:

"Failed to ping MapGuide Server. Perhaps it went offline. If it did, refresh the Site Explorer to create a new MapGuide session"

or something to that effect.

[jng]

Duplicate of #1723