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

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