Include errors in response object

[pehala]

Use case:
We use HttpX for testing and we would like to check if response returns error similar to how we check if the request was successful.status_code). That way we do not have to do two types of checks, assert for successful requests and try/catch for errors.

Proposal:
Have request.error() -> Exception and request.has_error() -> bool to request and raise_exception parameter to request which would switch if the error should be raised or be accessible only through the methods.