performance_notifier: strip useless info from the raise message
performance_notifier: strip useless info from the raise message

When raising:

* there's no point in printing "signature" because Ruby will
  provide a file and line number
* there's no need to print `payload` because we already state in words that it's
  empty (so we would always print `({})`)

airbrake/airbrake-ruby,airbrake/airbrake-ruby