Improve encryptedmedia exception messages.

To make this testable, this patch also makes `video-test.js` exception messages
visible in the test results.

BUG=270033

Review URL: https://codereview.chromium.org/133633002

git-svn-id: svn://svn.chromium.org/blink/trunk@164866 bbb929c8-8fbe-4397-9dbb-9b2b20218538