improve error handling when objects do not exist

[samtkaplan]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 75.98%. Comparing base (e1dcf76) to head (9eee16e).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/AzStorage.jl	73.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
- Coverage   80.00%   75.98%   -4.02%     
==========================================
  Files           1        1              
  Lines         445      458      +13     
==========================================
- Hits          356      348       -8     
- Misses         89      110      +21     

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