Elaborate error handling on ScriptLoader::finishLoading()

This is a follow up for r170702, which considered only about
dynamically added <script>. This change covers parser-added <script>.

TEST=adopt-node-crash.html
BUG=365229,364770
R=esprehn@chromium.org, eseidel@chromium.org

Review URL: https://codereview.chromium.org/248013002

git-svn-id: svn://svn.chromium.org/blink/trunk@172804 bbb929c8-8fbe-4397-9dbb-9b2b20218538