Fix md5sum error handling.
Fix md5sum error handling.

In https://chromium-review.googlesource.com/c/catapult/+/2170402, I
removed some sketchy code that make the md5 sum script always return
true (when the md5sum binary wasn't out of date), so now the md5sum
script actually returns a failure when it fails, which is good.

However, we were handling the wrong type of error that wasn't catching
the md5sum script failing.

Bug: chromium:1080856
Change-Id: Ia9126625e3eec3c173f1f6a6e12548f33ae11af1
Reviewed-on: https://chromium-review.googlesource.com/c/catapult/+/2197464
Commit-Queue: Michael Thiessen <592b72e1cff7dc8c6fa9a145a6d891fe715588c7@chromium.org>
Commit-Queue: Ben Pastene <e214258550f76fa23ac7c1d953424f3f468f5c6b@chromium.org>
Auto-Submit: Michael Thiessen <592b72e1cff7dc8c6fa9a145a6d891fe715588c7@chromium.org>
Reviewed-by: Ben Pastene <e214258550f76fa23ac7c1d953424f3f468f5c6b@chromium.org>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult