code: public String checkToken (BureauCredentials creds)
      {
          Bureau bureau = _bureaus.get(creds.clientId);
          if (bureau == null) {
              return "Bureau " + creds.clientId + " not found";
          }
          if (bureau.clientObj != null) {
              return "Bureau " + creds.clientId + " already logged in";
          }
          if (!creds.areValid(bureau.token)) {
              return "Bureau " + creds.clientId + " does not match credentials token";
          }
          return null;
      }
docs: Check the credentials to make sure this is one of our bureaus.
  @return null if all's well, otherwise a string describing the authentication failure