Fix JSON file download failure fallback

[Rylan12]

Follow-up to #14491

We want to actually use the old JSON file if we're falling back. Currently, we "fall back" by returning nothing which causes CaskLoader to assume that the API is not a viable option and skipping to loading from taps if they're available. Instead, we want to warn and then use the old version.

I think this will help with the weird cask loading errors we've been seeing. My best theory at the moment is that they happen when the API download fails and then it accidentally falls back to the taps.

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I think this will close #14483

[BrewTestBot]

Review period skipped due to critical label.

[apainintheneck]

This makes sense to me. I've actually been wondering if we could fall back to the cached JSON more often like when the command doesn't require the latest and greatest formulae/cask list. I assume that any commands that wouldn't work correctly with an out-of-date list can't be used with HOMEBREW_INSTALL_FROM_API anyway (like live check for example).

[MikeMcQuaid]

I think this needs tweaked a little as if the second JSON parse fails it won't be caught properly. I'll push t the PR.

[MikeMcQuaid]

I've actually been wondering if we could fall back to the cached JSON more often like when the command doesn't require the latest and greatest formulae/cask list.

Yeh, I think we could avoid checking e.g. more than once a minute for the newest things?