Better errors and responses!

Modifies send to accept decoders for both 2xx and non-2xx response bodies.

Adds a custom response type for which send returns a task that looks like:

type alias Response a =
  { data : a
  , status : Int
  , statusText : String
  , headers : Dict String String
  , url : String
  }

and introduces a new error type which will pass through a Response a in the case of non-2xx results:

type Error a
  = Timeout
  | UnexpectedPayload String
  | NetworkError
  | BadRequest (Response a)

This way you get built-in JSON decoding as well as response information for failed requests.