fix: guard against anomalous crt API responses

[TrebledJ]

I've been observing anomalous API responses from the crt API returning 404 and 503 status codes, which may trigger especially if we abuse(?) the API by repeatedly querying the same domain from the same IP. This change shouldn't affect most domains and users, but should be an added safeguard in case the API goes wonky.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91%. Comparing base (5be4993) to head (0541deb).
⚠️ Report is 13 commits behind head on dev.

Additional details and impacted files

@@          Coverage Diff          @@
##             dev   #3077   +/-   ##
=====================================
+ Coverage     91%     91%   +1%     
=====================================
  Files        437     437           
  Lines      37509   37511    +2     
=====================================
+ Hits       33925   33927    +2     
  Misses      3584    3584           

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