Fixed #22058 -- Refactored handlers and added a generic mechanism to deal with client and server errors.

**What is done**:

- Refactored handlers into generic ones.
- Deprecated original default views.
- Added generic views.

**Improvements Required**:
- Docs build have several warnings due to removing original handlers.
- A writeup for new views.

**Suggestions**:
- We could add two generic exceptions also: `HttpClientException` and `HttpServerException` to add onto the functionality.

I think `RequestContext` was omitted from at least `server_error` because if there's a problem with one of the context processors, you'll be unable to render to `server_error` view. You can probably read about the decisions in tickets.