Log which circuits failed during scan.

[NullHypothesis]

We should log the exit relays for which we were unable to create circuits during a scan. Perhaps this information should even be logged to disk, so it's easier to later find relays for which circuits failed repeatedly.

[s4chin]

I want to try this one. How do I go about this?

[NullHypothesis]

When a circuit to an exit relay cannot be created, it raises an exception here: https://github.com/NullHypothesis/exitmap/blob/master/src/exitmap.py#L394 I would keep track of all failed circuits, and print them once exitmap is finished.

[frankcash]

Hello, it's been a while since I've helped work on this project. I'd love to contribute another PR. Is there already a function that runs at the end of the "mapping" process or would one need to write this?