code: public static String getFailureDescription(ModelNode operationResult) {
          if (isSuccess(operationResult)) {
              return null;
          }
          if (operationResult != null) {
              final ModelNode descr = operationResult.get(FAILURE_DESCRIPTION);
              if (descr != null) {
                  return descr.asString();
              }
          }
          return "Unknown failure";
      }
docs: If the operation result was a failure, this returns the failure description if there is one.
  A generic failure message will be returned if the operation was a failure but has no failure
  description. A <code>null</code> is returned if the operation was a success.
  
  @param operationResult the operation whose failure description is to be returned
  @return the failure description of <code>null</code> if the operation was a success