Google App Script custom error on .withFailureHandler - google-sheets-api

Question

I would like to throw a custom error in a function called with google.script.run from code.gs so I could display adequate information in a side bar. So far I've tested the following code with no luck:
code.gs
function UserException(type, text) {
this.type = type;
this.text = text;
//this.stack = (new Error()).stack;
}
UserException.prototype = Object.create(Error.prototype);
UserException.prototype.constructor = UserException;
function assignRangeToTechnician(technician)
{
if(technician!=null)
{
//some code
}else
throw new UserException("Error","Technician was not selected");
}
sidebar.html
...
<script>
function btnSelectTech()
{
google.script.run
.withSuccessHandler(rangeSelected)
.withFailureHandler(techniciansMessage)
.assignRangeToTechnician(document.getElementById('selectTechnician').value);
}
function techniciansMessage(Message)
{
var outputMessage = document.getElementById('message');
//here is where I log the Message value
google.script.run.myLog("In techniciansMessage() - Message: " + Message);
if (Message == null)
outputMessage.innerHTML = "<p style='color:red;'>Error occured</p>";
else
if (Message.type == "Error")
outputMessage.innerHTML = "<p style='color:red;'>" + Message.text + "</p>";
else if (Message.type == "Message")
outputMessage.innerHTML = "<p style='color:#f3f3f3;'>" + Message.text + "</p>";
}
</script>
...
When I run the code the .withFailureHandler is called but the Message doesn't hold the proper value. When I log that message I read "Error: " as a content of a 'Message' parameter.
Could you please help?
Thank you.

Answer 1

You may refer with this SO thread. Try adding an error parameter to your function. Example:
google.script.run.withFailureHandler(function (error) {
showError(error, 'getMe');
}).getMe();
Additional reference which might help: https://github.com/google/google-apps-script-samples/blob/master/translate/Sidebar.js.html