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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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[bot]]

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