x/pkgsite: duplicate escaping on error page

[findleyr]

Via @rsc, who encountered the following broken escaping on pkgsite:

Repro steps:

1. Visit https://pkg.go.dev/rsc.io/tmp/nonexist
2. Click "Fetch" button.
3. Wait for the load to fail.

[findleyr]

Interestingly, the test appears to look for this escaping explicitly:
https://cs.opensource.google/go/x/pkgsite/+/master:internal/frontend/fetchserver/fetch_test.go;l=170;drc=ebe617b30c52774fccf9d36331cbff31e99aaf9e

[findleyr]

Hmm, I don't know if/how this ever worked. Perhaps @jba recalls?

fetch.ts uses new DOMParser().parseFromString(...), but that doesn't unescape:

[hyangah]

I guess this was fixed just by updating the pkgsite backend... last week.