Fix display of Dandiset validation errors

[jwodder]

Split off from #775

[codecov]

Codecov Report

Merging #777 (9774952) into master (7bba7ff) will decrease coverage by 0.17%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #777      +/-   ##
==========================================
- Coverage   85.08%   84.90%   -0.18%     
==========================================
  Files          59       59              
  Lines        6040     6041       +1     
==========================================
- Hits         5139     5129      -10     
- Misses        901      912      +11     

Flag	Coverage Δ
unittests	84.90% <0.00%> (-0.18%)	⬇️

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Impacted Files	Coverage Δ
dandi/dandiapi.py	92.46% <0.00%> (-0.39%)	⬇️
dandi/support/generatorify.py	0.00% <0.00%> (-16.10%)	⬇️
dandi/_version.py	45.96% <0.00%> (+1.75%)	⬆️

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[yarikoptic]

thank you!