Add information to Sentry error reports about what JavaScript files were
included on the page.

I have a hypothesis that Sentry issues like
https://sentry.io/organizations/hypothesis/issues/2528337318/events/e176f165836149b99d439fa71e69453c/?project=69811&query=is%3Aunresolved
might be caused by unwanted `<script>` tags injected by extension or the browser.
We try to block most unexpected scripts using strict
Content-Security-Policy settings, but extension and custom
browser-injected scripts may be able to bypass this.