title: Is it Possible to add custom validation like user exist using updater method in keystone js
body: <p>Is it Possible to add custom validation like  user exist using updater method in keystone js</p>
  
  <pre><code>updater.process(req.body, {
          flashErrors: true,
          fields: 'name, email, phone, enquiryType, message',
          errorMessage: 'There was a problem submitting your enquiry:',
      }, function (err) {
          if (err) {
              locals.validationErrors = err.error;
              console.log(err);
          } else {
              locals.enquirySubmitted = true;
          }
          next();
      });
  </code></pre>
  
  <p>Thanks</p>