Use async callbacks for custom error handler

**Is your feature request related to a problem? Please describe.**
Responses have to be available immediately on the custom error handler, but it can be callback-based just like request handlers, which allows for async responses, and request forwarding.

**Describe the solution you'd like**
Use callbacks instead of HttpResponsePtr returns.

**Describe alternatives you've considered**
Using ADD_METHOD_VIA_REGEX to capture everything that is not captured by file responses, API endpoints, WebSocket endpoints, etc.