Improve inspector/profiler/heap-snapshot-loader.html output when it fails

Also don't wait until snapshot is fully parsed in the worker when calculating canSaveToFile result.

BUG=330617
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/120153004

git-svn-id: svn://svn.chromium.org/blink/trunk@164330 bbb929c8-8fbe-4397-9dbb-9b2b20218538