Link checker false positives

[anjackson]

The link checker GitHub Action is hitting quite a few false positives. This seems to be due to two problems:

• Checks are too aggressive: if subsequent links refer to the same host, that host gets a rapid series of requests that can lead it to temporarily blocking the checker.
• Checks are too brittle: if a check fails with a potentially transient error (eg. 403, 500,...) the checker gives up. It would make sense to wait a little while and retry, just to rule out very short term issues. Ideally, we'd allow links to fail for a few days before flagging them, but this is much more difficult.

[anjackson]

Workaround is to not check some problematic links at treat 429s as 200s. I'll leave this open as it's not really a proper solution. There have been a couple of issues lodged with the underlying tool, which seems to think delaying only on 429 is sufficient. Perhaps I'll try that again.

[anjackson]

Okay, with a suitably large default timeout, that seems to work okay.