make file storage create directory upon instantiation

[atravitz]

Makes FileStorage behave more similarly to expected Pathlib behavior.
This also enables better error handling and guardrails for FileSystemWarehouse, as mentioned in OpenFreeEnergy/openfe#2155 (comment).

As written, this PR breaks existing behavior. Setting exist_ok=True by default would keep existing behavior. In my opinion, since ExternalStorage isnt used elsewhere yet, we should stick with the safer exist_ok=False option, but I can be convinced otherwise.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.65%. Comparing base (0b0edd9) to head (f96c37d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #827   +/-   ##
=======================================
  Coverage   98.65%   98.65%           
=======================================
  Files          43       43           
  Lines        2819     2820    +1     
=======================================
+ Hits         2781     2782    +1     
  Misses         38       38           

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