Show an error dialog when the backend throws an exception

[laurachiticariu]

In some circumstances, the XML generation or AQL compilation may throw an exception. Currently, the front-end does not show that exception. Instead, the front-end displays any results that were generated (from a subset of the nodes) and the user does not have an indication that there was a problem processing the rest of the nodes.

We would like the front-end to show a dialog with an exception, listing the full text of the exception for now.

In time, the UI will have enough front-end validation that will always prevent invalid JSON spec input to the backend, so the need for this error will become less and less over time.