Display apex error message in lightning component

Question

I am trying to display apex catch exception error message in lightning message. I have tried below in helper. but it didn't work. And let me know how to call this from Lightning component.

Helper:

    saveAction.setCallback(this,function(response){
                    if (response.getState() == "SUCCESS") {
                    component.set("v.isShow",false);
                    var opportunitId = response.getReturnValue();
                    window.location.href = "/"+opportunitId;
                    }
                });
                $A.enqueueAction(saveAction);

Lightning Controller:

    saveRecord : function(component, event, helper)
        {
            helper.helperSave(component, event);
        }

Answer 1

two things,

In your apex handle the exception like so

try {
    // your code here
}
catch (Exception e //if you know what type of error it is going to throw you can be more specific ie. NullPointerException e) {
    // "Convert" the exception into an AuraHandledException
    throw new AuraHandledException(e.getMessage());    
}

Now you have to handle the error in the javascript controller

  saveAction.setCallback(this,function(response){
      let state = response.getState();          
      if (state == "SUCCESS") {
        component.set("v.isShow",false);
        var opportunitId = response.getReturnValue();
        var navEvt = $A.get("e.force:navigateToSObject");
        navEvt.setParams({
           "recordId": opportunityId
        });
        navEvt.fire();
       } else if(state = "ERROR"){
           errorMsg = response.getError()[0];
           let toastParams = {
             title: "Error",
             message: errorMsg, // Default error message
             type: "error"
            };
           let toastEvent = $A.get("e.force:showToast");
           toastEvent.setParams(toastParams);
           toastEvent.fire();
          }
        });
        $A.enqueueAction(saveAction);
    }

Also use the built in object redirect to navigate to the opportunity.

var navEvt = $A.get("e.force:navigateToSObject");
navEvt.setParams({
  "recordId": opportunityId
});
navEvt.fire();

Answer 2

First, throw an aura handled the exception of the controller.

throw new AuraHandledException('something terrible happened!');

Then handle it in the aura component controller or component if your following best practices. Check if the response state was successful by checking the response state. If there is an error, it will be an array. Due to the aura framework using the LockerService API for security every object is a Proxy Object, that is why JSON.parse and JSON.stringigy is may be needed.

const state = response.getState();
if(state === 'ERROR'){
    const errorMsg = response.getError()[0].message));
}