Improve Internals exception messages.

Who tests the testers? No one, I assume. Let's see what the bots say.

BUG=270033

Review URL: https://codereview.chromium.org/138283003

git-svn-id: svn://svn.chromium.org/blink/trunk@165737 bbb929c8-8fbe-4397-9dbb-9b2b20218538