change error in warning visium_hd

[melonora]

Errors are raised when one of the image files is not there. This PR changes it into a warning.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 45.50%. Comparing base (54da345) to head (280b79f).
Report is 189 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatialdata_io/readers/visium_hd.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #206      +/-   ##
==========================================
+ Coverage   35.91%   45.50%   +9.58%     
==========================================
  Files          19       22       +3     
  Lines        1715     2147     +432     
==========================================
+ Hits          616      977     +361     
- Misses       1099     1170      +71     

Files with missing lines	Coverage Δ
src/spatialdata_io/readers/visium_hd.py	18.78% <0.00%> (-2.96%)	⬇️

... and 4 files with indirect coverage changes