Correct error reporting in run-bindings-tests

Currently run-bindings-tests:
* has dead code (diff results in exception, so checking output never hit)
* has redundant code (always want to print output of commands)
* ...and lacks clear distinction of script errors vs. diffs

This arose b/c I noticed that I never saw the 'FAIL' line, as that was
dead code.

This fixes these problems.

Review URL: https://codereview.chromium.org/73513003

git-svn-id: svn://svn.chromium.org/blink/trunk@162109 bbb929c8-8fbe-4397-9dbb-9b2b20218538