Fix API crash caused by socket closed errors

[nellh]

A request for a deleted dataset tree object could cause a crash due to unhandled errors opening the repository. This changes both the worker and the API to handle this situation better and return more sensible HTTP 404 errors when requesting trees or datasets that do not exist.

[codecov]

Codecov Report

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

Comparison is base (86ef124) 65.08% compared to head (7ce531f) 72.61%.
Report is 2 commits behind head on master.

Files	Patch %	Lines
services/datalad/datalad_service/handlers/files.py	76.47%	4 Missing ⚠️
services/datalad/datalad_service/handlers/tree.py	62.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2959      +/-   ##
==========================================
+ Coverage   65.08%   72.61%   +7.53%     
==========================================
  Files         378       43     -335     
  Lines       24309     1607   -22702     
  Branches      884        0     -884     
==========================================
- Hits        15822     1167   -14655     
+ Misses       8487      440    -8047     

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