MgException messages are lost in mapagent

[jng]

The recent RFC to unify all exceptions to a single MgException has subtly broken error message display in the mapagent.

Known issues:

• The exception type MgException is being set as the status message. This should now be the code of the caught MgException
• It is failing to resolve the localized message when displaying the exception message. Actually it's just that the failure to retrieve the localized message for exceptions thrown by the implementation of #2864 that is masked by a separate error that is not being surfaced.

[jng]

In 10040:

Use the exception code as the http status message and fix MgPermissionDenied exceptions being thrown with no arguments

Fixes #2866