No test because in practice these errors never occur.

Change-Id: I11c77893ae931fc621c98920cba656790d18ed93
Reviewed-on: https://go-review.googlesource.com/34811
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>