Fail gracefully when we can't access a web resource

[jag1g13]

In lowfat.validator.online_document we check that a resource (e.g. blog post) is reachable. On 2020-11-26 something weird was happening with the network which caused all of these checks to fail. This caused requests which rely on these to Error 500.

From the server only ed.ac.uk was pingable, everything else I tested failed - hence confidence that this is a network issue.

[jag1g13]

Couldn't replicate with a resource that did not exist - could potentially arise only with certain HTTP response codes, or when a resource exists but times out?