Add dedicated HTTP404Error exception class

[jwodder]

This cuts down on a lot of simple repetition by adding a dedicated HTTP404Error exception class subclassing requests.HTTPError that is used specifically for 404 errors.

[codecov]

Codecov Report

Merging #908 (3b69708) into master (de4ff71) will increase coverage by 0.05%.
The diff coverage is 65.71%.

@@            Coverage Diff             @@
##           master     #908      +/-   ##
==========================================
+ Coverage   86.74%   86.79%   +0.05%     
==========================================
  Files          61       61              
  Lines        7447     7424      -23     
==========================================
- Hits         6460     6444      -16     
+ Misses        987      980       -7     

Flag	Coverage Δ
unittests	86.79% <65.71%> (+0.05%)	⬆️

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Impacted Files	Coverage Δ
dandi/dandiapi.py	89.69% <62.50%> (+1.30%)	⬆️
dandi/exceptions.py	100.00% <100.00%> (ø)
dandi/dandiarchive.py	82.37% <0.00%> (-2.05%)	⬇️

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[yarikoptic]

LGTM, thank you!