code: public void addErrorPage(final WebAppErrorPage errorPage) {
  		NullArgumentException.validateNotNull(errorPage, "Error page");
  		if (errorPage.getErrorCode() == null
  				&& errorPage.getExceptionType() == null) {
  			throw new NullPointerException(
  					"At least one of error type or exception code must be set");
  		}
  		errorPages.add(errorPage);
  	}
docs: Add an error page.
  
  @param errorPage to add
  @throws NullArgumentException if error page is null or both error type and exception code
  is null