Give "delete" a --skip-missing option

[jwodder]

Closes #592.

[codecov]

Codecov Report

Merging #594 (4df5c02) into master (4e80384) will increase coverage by 0.03%.
The diff coverage is 77.96%.

@@            Coverage Diff             @@
##           master     #594      +/-   ##
==========================================
+ Coverage   81.23%   81.26%   +0.03%     
==========================================
  Files          62       62              
  Lines        6601     6651      +50     
==========================================
+ Hits         5362     5405      +43     
- Misses       1239     1246       +7     

Flag	Coverage Δ
unittests	81.26% <77.96%> (+0.03%)	⬆️

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Impacted Files	Coverage Δ
dandi/delete.py	89.10% <60.00%> (-7.14%)	⬇️
dandi/cli/cmd_delete.py	81.81% <66.66%> (+1.81%)	⬆️
dandi/tests/test_delete.py	100.00% <100.00%> (ø)
dandi/_version.py	45.96% <0.00%> (+1.75%)	⬆️

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[yarikoptic]

Thank you @jwodder! Looks good and will test as deployed