Figure out better way to handle status codes for network errors

[Mr0grog]

In #1184, I added support for recording versions with network errors (instead of valid HTTP responses), but wound up leaving status as nil in that case. I’d originally planned to set it to 0 as a clear indicator that there was no status code (since we have lots of records of unknown status code from older sources). But we‘ve got some validations to keep them in the standard 100–599 range, and I’m not sure what other code might be expecting these to always be valid.

This issue also caused me to treat network errors as 404 when calculating the page’s overall status, which works, but is definitely not ideal.

Needs some thinking as to what (if anything) should be done here.