DevTools: bring back exception handling in URL decoding

The exception handling was added in r177449, but then was accidentally
removed via r183686. This patch brings the test and functionality back.

R=vsevik

Review URL: https://codereview.chromium.org/661983003

git-svn-id: svn://svn.chromium.org/blink/trunk@183901 bbb929c8-8fbe-4397-9dbb-9b2b20218538