Add Response::error_for_status method to simplify checking responses

[adamreichold]

Closes #56

[sbstp]

Is it worth also adding the ref variant? https://docs.rs/reqwest/0.10.4/reqwest/struct.Response.html#method.error_for_status_ref

[adamreichold]

Is it worth also adding the ref variant?

I did wonder about that but came to the conclusion that the by-value version provides more safety as one cannot access a failed response after turning it into a Result. Since the by-ref variant could just as well be open coded, I preferred to keep it safe and simple.

That said, I have no real problem with adding that if you would like to have it.

[sbstp]

Fine like this. If someone wants the feature we can iterate later.