Show an error dialog in the case that the report already exists.

[ccanash]

When this happen in the app:

1. Identify a AT/browser/test plan combination with a report on the reports page.
2. Add a run to the test queue for that test plan and AT/browser combination.

The result is:

1. The report is removed from the report site.
2. The test queue has previous results for that AT/browser combo and a "Mark as Final" button.

In #870 a new behavior is requested for the result, while we identify the best approach for it we will deploy a temporary solution that shows an error dialog in the case that the report already exists.