This permits specifying an ErrorHandler to customize the RoundTrip
error handling if the backend fails to return a response.

Fixes #22700
Fixes #21255

Change-Id: I8879f0956e2472a07f584660afa10105ef23bf11
Reviewed-on: https://go-review.googlesource.com/77410Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>