Errors from http.Client already includes the URL in the message.

Fixes #18754

Change-Id: I65fc25a8f3aa6a2d4627aac3fb47eed8d3c4151a
Reviewed-on: https://go-review.googlesource.com/35650Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>