How to handle errors and success at the same time on a Google Apps Script Webapp

I am wondering how the client-server communication with Google Apps Script Webapps works. In the html page I call my functions with

google.script.run.doSomething();

and I can add a

withFailureHandler(onFailure);

withSuccessHandler(onSuccess) but I can't add both..

So when I want to call a server-side function I normally want to handle the response in the UI ?differently if it is a success than a failure, right? But I want to handle both right? So why do I have to choose between one of them?

Also another problem is that I could not find any information about the actual errors you can have on the server side so that your withFailureHandler(onFailure); can handle them. Can I just do a throw new Error("everything is broken - tell that the user")? If there are permission errors, are they handled? How do I generate an error on the server side so that I can handle it on the client side properly?

Solution

It is true that the documentation does not specify it clearly, but you can implement both a success and error handler simultaneously

Sample:

<html>
  <head>
   <base target="_top">
    <script>
      google.script.run.withSuccessHandler(onSuccess).withFailureHandler(onFailure).getUnreadEmails();
     function onSuccess(numUnread) {
       var div = document.getElementById('output');
       div.innerHTML = 'You have ' + numUnread  + ' unread messages in your Gmail inbox.';
     }
     function onFailure(error) {
       var div = document.getElementById('output');
       div.innerHTML = "ERROR: " + error.message;
     }
    </script>
  </head>
  <body>
    <div id="output"></div>
  </body>
</html>

UPDATE

To simulate a failure based on the documentaiton sample, change the working code.gs part

from

function doGet() {
  return HtmlService.createHtmlOutputFromFile('Index');
}

function getUnreadEmails() {
  return GmailApp.gotInboxUnreadCount();
}

function doGet() {
  return HtmlService.createHtmlOutputFromFile('Index');
}

function getUnreadEmails() {
  return GMailApp.gotInboxUnreadCount();
}

After deploying the WebApp, the html will output: