Check for return value on dataset delete

[BenGalewsky]

Problem

The delete command in the dataset cli tests for a True return value from the ServiceXAdapter. Unfortunately, the delete_dataset method doesn't return anything.

Approach

Add a check for the stale property in the value returned from the server and return that from the method

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.05%. Comparing base (c4c8562) to head (1df9c49).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #529      +/-   ##
==========================================
+ Coverage   88.04%   88.05%   +0.01%     
==========================================
  Files          26       26              
  Lines        1656     1658       +2     
==========================================
+ Hits         1458     1460       +2     
  Misses        198      198              

Flag	Coverage Δ
unittests	88.05% <100.00%> (+0.01%)	⬆️

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[ponyisi]

Looks good

[ponyisi]

Resolves #528, correct?