Improve error reporting in GUI
Moved from Bugzilla:
https://bugs.eclipse.org/bugs/show_bug.cgi?id=560880
Error reporting is largely limited to the console or the logging facilities for many operations.
Issue #560848 in Bugzilla changed the internal APIs for metamodel registration so that the problems could bubble up to the API, but a few other internal IMetaModelUpdater methods still rely on boolean success/failure flags instead of rethrowing the appropriate exceptions.
This includes removing metamodels, and managing derived/indexed attributes.