handleError = (username, error) => {
    functions.logger.error({ User: username }, error);
    res.sendStatus(500);
    return;
  }
Authenticate the provided credentials returning a Firebase custom auth token.
`username` and `password` values are expected in the body of the request.
If authentication fails return a 401 response.
If the request is badly formed return a 400 response.
If the request method is unsupported (not POST) return a 403 response.
If an error occurs log the details and return a 500 response.