Retry on 504 errors (and others?)

[benoit74]

We have some fandom pages which are failing with a 504 error and stopping the run. See e.g. https://farm.openzim.org/pipeline/7dd8a1d4-7759-4f0a-8252-58c2a33ced68/debug

Trying these URLs manually, it looks like they sometimes work on second or third try, i.e. the problem is only that upstream was still processing, but when retrying the data is probably fresh inside the DB and hence achieves to make it before the timeout. But this is only "sometimes".

We should be cautious with that (to not spend too much time retrying) but retrying these pages could maybe help.

Somewhat related to #2152 (handling of 429) and #1617 as maybe kinda a duplicate of #1596