Validate uploads before digesting

[jwodder]

Closes #492.

[codecov]

Codecov Report

Merging #495 (5e880b2) into master (089101e) will not change coverage.
The diff coverage is 81.25%.

@@           Coverage Diff           @@
##           master     #495   +/-   ##
=======================================
  Coverage   82.73%   82.73%           
=======================================
  Files          59       59           
  Lines        6054     6054           
=======================================
  Hits         5009     5009           
  Misses       1045     1045           

Flag	Coverage Δ
unittests	82.73% <81.25%> (ø)

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Impacted Files	Coverage Δ
dandi/upload.py	75.71% <81.25%> (ø)

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[yarikoptic]

Looks kosher, thank you @jwodder !