fix(server): Add missing dataset read checks

[nellh]

Fix missing read checks on file and object APIs.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.06%. Comparing base (ead8d93) to head (e4326d3).
⚠️ Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
packages/openneuro-server/src/handlers/datalad.ts	0.00%	17 Missing ⚠️
packages/openneuro-server/src/routes.ts	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4060      +/-   ##
==========================================
+ Coverage   48.89%   49.06%   +0.16%     
==========================================
  Files         686      686              
  Lines       38075    38137      +62     
  Branches     1897     1909      +12     
==========================================
+ Hits        18616    18711      +95     
+ Misses      19295    19256      -39     
- Partials      164      170       +6     

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[codecov]

Bundle Report

Bundle size has no change ✅