code: public IValidator appendError(IValidator validator) {
  		if (validator == null) return this;
  		if (! (validator instanceof WebValidator) ) {
  			throw new IllegalArgumentException("Must WebValidator.");
  		}
  		
  		WebValidator valid = (WebValidator) validator;
  		if (valid.isSuccess())
  			return this;
  		if (this.success) {
  			this.msg = valid.msg;
  			this.success = valid.success;
  		} else {
  			msg.append("<br>").append(valid.getMsg());
  		}
  		return this;
  	}
docs: Only append Error. Not append validator if success.
  @return <code>this</code>