Exhaust all possibilities for each exception type before bailing, in bugger functions.

Currently, we generate a list of py_files that can be modified. Then choose relevant nodes. Then choose one to manipulate. But if that manipulation fails, we simply return without generating a bug.

We should keep looping over all possible nodes until we successfully generate a bug, or run out of relevant nodes to consider. If we can't generate that kind of exception, it should be removed from requested_bugs. We should also make sure we're still reporting accurately which bugs we were able to introduce, and which we were not able to introduce.

- [ ] Failing test
- [ ] Implement support for `-e ModuleNotFoundError`
- [ ] Passing test
- [ ] Repeat for other exception types
- [ ] `-e AttributeError`
- [ ] `-e IndentationError`
- [ ] Update contributing docs, where logical flow is explained?
- [ ] Merge
- [ ] Release
- [ ] Announce, as better public bugmaking algorithm.