Properly handle failed requests to retrieve the schema

[danlamanna]

We ran into this a week or so ago when github had their outage. This should provide a clearer exception than the JSONDecodeError we saw.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.75%. Comparing base (6fc16b8) to head (dd8a837).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #251   +/-   ##
=======================================
  Coverage   97.75%   97.75%           
=======================================
  Files          16       16           
  Lines        1739     1741    +2     
=======================================
+ Hits         1700     1702    +2     
  Misses         39       39           

Flag	Coverage Δ
unittests	97.75% <100.00%> (+<0.01%)	⬆️

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[candleindark]

LGTM. Thank @danlamanna. @yarikoptic, merge? The test-dandi-cli (ubuntu-latest, 3.8, dandi-devel, master) is hung. I couldn't restart it.

[yarikoptic]

FWIW, that test stall is not new:

• test_nwb2asset_remote_asset timed out in test (ubuntu-latest, 3.8, dandi-api) dandi-cli#1450

I restarted that run just in case.