How to transform a HttpResponse into a HttpErrorResponse

I have a web service returning custom errors inside a 200 Response, in a classic errors object.

This breaks my workflow and patterns, where I usually intercept proper HTTP error status codes and go on with handling the various error cases from there.

I need to transform these custom errors into proper HttpErrorResponse errors via an interceptor, so that I can then intercept those now-proper errors and continue with my application per usual best practices.

Things I can't do:

• Have the web service modified so that it returns proper HTTP errors
• Have another web service in the middle to handle the transformation before the Response hits the client application

What I'd like to do:

• Make the following Stackblitz example work. The part that needs to be fixed is in interceptors/custom-error.interceptor.ts
  https://stackblitz.com/edit/angular-pnv63p

So that when in the near future I'll be working on several other applications that use the same unmodifiable web service, I can just drop the CustomErrorInterceptor in and call it a day.