Add a cancel button on test discovery failure, so we can cleanly abort

[stephenmcd]

Hey Russell,

This is really cool, thanks for releasing it.

One issue I had was that I haphazardly first ran the django module in a directory without a Django project. This triggers an error dialogue with a retry button which is all well and good, but there was no obvious way to quit out of this scenario - even Ctrl C on the terminal failed and I couldn't escape without kill -9ing the process.

My change adds a cancel button to the error dialogue, which gets overridden for TestLoadErrorDialog to shut down the whole app.

Thanks again!

Steve

[thsutton]

Does this not lead to an error as the app ends?

_tkinter.TclError: can't invoke "toplevel" command:  application has been destroyed

[stephenmcd]

Yes I did receive that error, but wasn't sure how to surpress it. First time touching tkinter here :-)

[thsutton]

I had the same response. :-)