Bogus content seems to work correctly and result in a dialog with a stacktrace.  But I'm seeing a completely blank dialog (no title, no content) when I stop fcrepo4, when I do any of the following: add object, delete object, delete datastream/object, sparql update, or start transaction.  Stopping the repo seemed like the easiest way to simulate failure for the core add/delete/update operations -- but maybe the error handling here is assuming a response from the repo, not total failure?.