If generating ptrace definitions for a particular pair fails for any
reason, a success message would still be shown. Avoid it in case there
was a failure.

Change-Id: Ica59c888586c055e8ef333a0db00bde2c4d15145
Reviewed-on: https://go-review.googlesource.com/73874
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>