UI: improve crash reporting

- For Analytics, encodes the crash as a page url.
  This is because GA4 truncates fields to 100 chars,
  but allows 1000 chars for the url. We are a SPA and
  don't need the URL thankfully.
- Gets rid of the duplicate message (once in the
  title, once in the top of the stack)
- Normalizes detection of stack function vs url on all
  major browsers.
- Also fixing python/tools/check_imports.py which doesn't
  seem to work with python 3.6.8 shipped on my gMac.

Change-Id: Idef3a65309cd49e232682075ae2da593f3bb54d0