Increase retries & wait times for API errors

[jwodder]

Closes #714.

Tenacity does not seem to support indefinite retrying for only one type of error.

[codecov]

Codecov Report

Merging #716 (2009d09) into master (b084c62) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #716   +/-   ##
=======================================
  Coverage   84.06%   84.06%           
=======================================
  Files          59       59           
  Lines        5661     5661           
=======================================
  Hits         4759     4759           
  Misses        902      902           

Flag	Coverage Δ
unittests	84.06% <100.00%> (ø)

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Impacted Files	Coverage Δ
dandi/dandiapi.py	90.93% <100.00%> (ø)

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