Fix Junit Reporter Crashes

__What__

There was a crash where the reporter was trying to mutate an immutable dictionary property on NSError. I changed the Errors dictionary that the reporter uses to generate the xml after it completes the test run to just use an NSString instead. Currently, we were only using the string key anyways, so we get better reporting on test failures by tacking its context in there.

__How__

- [ ] Make sure failing tests don't crash anymore. (they will still crash for steps with no error)
- [ ] Code Review.