Email Notification of a Failure
If you want to be notified by email or an alert pop-up when a webhook fails to send or receives an error response, you can set up an additional Event Trigger on the Export Adaptor Log with a condition set to trigger on receiving an error response status code (or whatever is the most appropriate trigger condition for your integration).
Troubleshooting
Trigger Condition Report
While testing an Event Trigger, it’s recommended to validate the trigger conditions by using the Event Trigger Report run from the modified record to check whether the trigger condition is met prior to saving the record. You can find this on the Action Menu - Document
Test by Creating a Mock Web Service using Postman
There are many ways to create a mock Web service for testing. One of the easiest is to use tools such as Postman (https://www.postman.com/) or WireMock (https://wiremock.org/).