bug-1902001: fix source highlight with bad urls

[willkg]

If the url provided isn't a valid url, treat it as if the document doesn't exist and return an HTTP 404.

To test:

> curl "http://localhost:8000/sources/highlight/?url=https://crash-stats.allizom.org\[@a.xxx.org/?"

Compare that with:

> curl "https://crash-stats.allizom.org/sources/highlight/?url=https://crash-stats.allizom.org\[@a.xxx.org/?"

(You need the \[ because otherwise curl thinks the [ means something and kicks up curl: (3) bad range specification in URL position 78:.)

[willkg]

Thank you!