Tests may want to look at the output from running a command, even if it
fails (e.g. with a non-zero return code). Provide a means to obtain this.

Another approach would be to return a class object containing both the
output and the exception, but I'm not sure if that would result in a lot
of refactoring.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Teddy Reed <teddy.reed@gmail.com>