[Bug: T362915] Show error message on unsupported URLs

The extension silently fails on URLs like https://chromewebstore.google.com/detail/wikipedia-citation-needed/kecnjhdipdihkibljeicopdcoinghmhj which is confusing to users. This change adds a bad URL function which shows an error message.

Error looks like

There are also random formatting changes here because it's formatting on save and our formatting settings aren't in sync.

It should probably also show this error for chrome:// urls but I think that's less important.