Don't try to truncate payloads with 'trace' if that key doesn't exist.
Rescue Rollbar::Notifier#process_payload and show error log message.
Process safely the payload when using async handlers. It will report internal errors.
Merge pull request #196 from rollbar/fix-frames-truncation-strategy

Don't try to truncate payloads with 'trace' if that key doesn't exist.