Is it possible to trigger an alert based on the presence of web page javascript errors? Typically these errors would be only be visible when examining a web page from a browser's Developer Tools -> Console views.
You would probably have to write a custom script to parse the output and embed that in a scripting component monitor.
The built in HTTP monitor is looking for text in the actual rendered page rather than data buried behind the scenes in the console.
That being said, I may not fully understand your use case. If you could elaborate a little more then perhaps myself or someone else
here could offer some additional advice.