Return error if no tags in bucket

[smallhive]

Closes #898.

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (f76190c) 33.68% compared to head (7d0b0e3) 33.66%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
api/layer/tagging.go	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #924      +/-   ##
==========================================
- Coverage   33.68%   33.66%   -0.02%     
==========================================
  Files          64       64              
  Lines       10834    10839       +5     
==========================================
  Hits         3649     3649              
- Misses       6777     6782       +5     
  Partials      408      408              

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