title: Custom exception name instead of UserLambdaValidationException
body: <p>I use the <code>Pre sign-up</code> trigger of Cognito to validate the user registration. If some validation fails the function responses with a custom error. The client only receive the <code>UserLambdaValidationException</code> as <code>ErrorCode</code>.</p>
  
  <p>Is there a way to receive the custom error name instead?</p>
  
  <p>Current using sample:</p>
  
  <pre><code>exports.handler = function(event, context, callback) {                
    function AccountAlreadyExistsError(message) {
      this.name = "AccountAlreadyExistsError";
      this.message = message;
    }
    AccountAlreadyExistsError.prototype = new Error();
  
    const error = new AccountAlreadyExistsError("Account is in use!");
    callback(error);
  };
  </code></pre>
  
  <p>I want to get <code>AccountAlreadyExistsError</code> in our client instead of <code>UserLambdaValidationException</code>.</p>