If an error occurs, delete the newly created directory.

Change-Id: I97e960d24af32f027fb23d5ce3f61959e73f6fb0
Reviewed-on: https://go-review.googlesource.com/115715
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>