net/http: if context is canceled, return its error

net/http: if context is canceled, return its error

This permits the error message to distinguish between a context that was
canceled and a context that timed out.

Updates #16381.

Change-Id: I3994b98e32952abcd7ddb5fee08fa1535999be6d
Reviewed-on: https://go-review.googlesource.com/24978
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>