[blinkpy] Log errors when trying to kill non-existent processes

More logging in hope of shedding some light on issue 929413
(content_shell outliving run_web_tests).

Bug: 929413
Change-Id: I47117a85a083158d13527a535dab312774220001
Reviewed-on: https://chromium-review.googlesource.com/c/1474630
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632388}
1 file changed