Add error boundaries and exception tracking to the website

[bvaughn]

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Part 1: Gracefully handle exceptions

If an error occurs, we should show some kind of fallback UI.

We already handle specific types of errors like failing to load Babel or Algolia libs from CDN, but a more general-purpose error boundary would be nice to handle other unexpected errors.

Part 2: Track exceptions

We should also log/track errors as well. Maybe we could use GA's exception tracking for this?