change error handling for /curation REST errors

[bkmartinjr]

Fixes #606

Changes:

• if the response from the upstream curation API is suspect, error out. Do not silently ignore and move on.
• where we were logging as an Error, but it was really a Warning, change log level
• raise RuntimeError in cases of intentionally "bailing" rather than raising AssertionError (save that for assertions)
• fix unit test to match new (expected) behavior

[codecov]

Codecov Report

Merging #608 (b44a463) into main (33084b8) will decrease coverage by 0.03%.
The diff coverage is 63.63%.

@@            Coverage Diff             @@
##             main     #608      +/-   ##
==========================================
- Coverage   88.10%   88.07%   -0.03%     
==========================================
  Files          62       62              
  Lines        3740     3741       +1     
==========================================
  Hits         3295     3295              
- Misses        445      446       +1     

Flag	Coverage Δ
unittests	88.07% <63.63%> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
.../cellxgene_census_builder/build_soma/build_soma.py	91.74% <0.00%> (ø)
...rc/cellxgene_census_builder/build_soma/manifest.py	94.02% <50.00%> (-1.36%)	⬇️
...src/cellxgene_census_builder/build_soma/anndata.py	86.90% <100.00%> (ø)
...ls/cellxgene_census_builder/tests/test_manifest.py	100.00% <100.00%> (ø)

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