[BF] Small fixes to latest cli commands

[jsheunis]

Just a couple of basic fixes and additional tests to fix issues that @mslw encountered during usage

• closes Validate won't work without a dataset #330
• closes catalog_set yields both "ok" and "impossible" #331

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.04% 🎉

Comparison is base (ae17248) 86.14% compared to head (e0d7c28) 86.19%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #333      +/-   ##
==========================================
+ Coverage   86.14%   86.19%   +0.04%     
==========================================
  Files          43       43              
  Lines        2700     2709       +9     
==========================================
+ Hits         2326     2335       +9     
  Misses        374      374              

Flag	Coverage Δ
unittests	79.07% <100.00%> (+0.07%)	⬆️

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Files Changed	Coverage Δ
datalad_catalog/set.py	100.00% <100.00%> (ø)
datalad_catalog/tests/test_set.py	100.00% <100.00%> (ø)
datalad_catalog/tests/test_validate.py	100.00% <100.00%> (ø)
datalad_catalog/validate.py	93.54% <100.00%> (+0.10%)	⬆️

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