make `Response#toError()` have a more meaningful `message`

Now it includes the .method and .path of the Request object, which
should aid in debugging when these kinds of HTTP requests fail.

Also the Error object now includes additional `method` and `path` fields.
Merge pull request #237 from TooTallNate/update/better-toError-message

make `Response#toError()` have a more meaningful `message`