Failing to create the output file would give confusing errors such as:

	cover: invalid argument

Also do out.Close() even if Execute() errored.

Fixes #17951.

Change-Id: I897e1d31f7996871c54fde7cb09614cafbf6c3fc
Reviewed-on: https://go-review.googlesource.com/33278Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>