code: protected Diagnostic createMandatoryDiagnostic() {
  		String errorMessage = getComponentModel().errorMessage;
  		String msg = errorMessage == null ? InternalMessages.DEFAULT_VALIDATION_ERROR_MANDATORY : errorMessage;
  		return createErrorDiagnostic(msg, this);
  	}
docs: <p>
  This method is called by validateComponent to create the mandatory diagnostic error message if the mandatory
  validation check does not pass.
  </p>
  <p>
  Subclasses may override this method to customise the message, however in most cases it is easier to supply a
  custom error message pattern to the setMandatory method.
  </p>
  
  @return a new diagnostic for when mandatory validation fails.