Multithreaded error handling in storage

[MaxiBoether]

If a thread in the get implementation throws an error, the storage currently dies because C++ does not propagate exceptions to the caller. With this change, we're able to return a gRPC error instead. Also, we throw now in case the file path is empty.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.50%. Comparing base (0a511bb) to head (e07f0ab).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #524      +/-   ##
==========================================
+ Coverage   82.43%   82.50%   +0.06%     
==========================================
  Files         214      214              
  Lines       10039    10039              
==========================================
+ Hits         8276     8283       +7     
+ Misses       1763     1756       -7     

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[github-actions]

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