[python] Throw error in SOMAVFSFilebuf.open on file-not-found

[nguyenv]

Issue and/or context:

[sc-61909]

Changes:

Fix misleading error by throwing an explicit error in SOMAVFSFilebuf.open if the file does not exist.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.27%. Comparing base (ac7407e) to head (e65943b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3565      +/-   ##
==========================================
+ Coverage   86.22%   86.27%   +0.04%     
==========================================
  Files          55       55              
  Lines        6410     6410              
==========================================
+ Hits         5527     5530       +3     
+ Misses        883      880       -3     

Flag	Coverage Δ
python	86.27% <ø> (+0.04%)	⬆️

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Components	Coverage Δ
python_api	86.27% <ø> (+0.04%)	⬆️
libtiledbsoma	∅ <ø> (∅)