Response::error is a footgun

[jyn514]

Response::error("Error", 200) is perfectly valid and is not correct at all.

Actually, looking at the implementation it would be fine if we just renamed this to Response::with_status. @nilslice what do you think? We could still have Response::internal_error or Response::request_error convenience wrappers if you think they're helpful.

(This may become a moot point after implementing #13.)

[nilslice]

Yea I think renaming this to Response::with_status is fine. I like having an error method though for spot-checking code, making it really easy to know immediately that a certain branch is returning a failure indicator.

Feel free to rename, and if you want to, we could add a Response::error method which verifies that the code is within a true-to-spec error range. 400 - 599 would be a good start.

[nilslice]

For now I've updated Response::error so it is only valid for error-related status codes. Adding a with_status method is a good idea too and I will probably include it along with some other clean-up work.