Status page: do not error if migration details can't be fetched; show zeros instead

[jaimergp]

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Closes #2389. If the URLs are missing, all migrations will have progress 0. See screenshot for simulated failure mode (I changed the URLs in constants.js locally for this):

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[jaimergp]

This fixes it, but if you click on the errored migration, it confusingly takes you back to the status page. Is that too bad? We can throw a 404-like error, but not sure if that's useful. Maybe I'll try to remove the link if no details were fetched...

[beckermr]

Maybe we can mark the row with (failed to load) after the migration name and not make the title a link at all?

[jaimergp]

Check last commit, we had the same thought :)

[beckermr]

LGTM! thank you!