function _gracefail(errorCode, quiet) {
  if (quiet) {
    logger.debug(RUM_ISSUES[errorCode])
  } else {
    logger.warn(RUM_ISSUES[errorCode])
  }
  return '<!-- NREUM: (' + errorCode + ') -->'
}
Gracefully fail.

Output an HTML comment and log a warning the comment is meant to be
innocuous to the end user.

@private
@see RUM_ISSUES
@param {number} errorCode Error code from `RUM_ISSUES`.
@param {boolean} [quiet] Be quiet about this failure.
@returns {string} HTML comment for debugging purposes with specific error code