function reformatError(logLine) {
  logLine['error.message'] = truncate(logLine.err.message)
  logLine['error.stack'] = truncate(logLine.err.stack)
  logLine['error.class'] = logLine.err.type
  delete logLine.err
}
Truncates the message, stack and class of error
and reassigns to a `error.*` keyspace. Also removes the `err`
key on a log line

@param {object} logLine log line