Custom threshold for Kingfisher Collect

[jpmckinney]

One third of spain_zaragoza's requests are 404. This has been consistent in every month since we started tracking (Jun, Jul, Aug).

Should we add logic to the registry to allow setting a higher threshold for individual publications?

Otherwise, spain_zaragoza will not be updated: open-contracting/kingfisher-collect#1155 (comment)

[jpmckinney]

If we add this, then we can increase the default threshold, since most datasets can achieve an error rate of 0% or less than 1%.

[jpmckinney]

Noting that we can perhaps reduce the error rate by defining is_http_error_expected in relevant spiders.

We should add a note about this in the new field's help message, as a reminder.