This preserves the promise that Do will return the context's error.

Fixes golang/go#16381

Change-Id: I0db49b175736a695199b38819b4ff97b83d9c5ed
Reviewed-on: https://go-review.googlesource.com/24977
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>