Comment 4 for bug 1528264

Aleksandr, the issue should be fixed in Nailgun code.
The problem is that Nailgun returns plain text response in the case. But UI expects to get json in response with the following format: {message: <original_text>}.

UI handles json response only because it can be an arbitrary string in plain text response, for example it can be traceback and UI should not display it.

I also would not say that this issue is related to some another ticket, because I guess there are many places where Nailgun returns plain text response.