Added exception when file exists and overwrite=false

[rozyczko]

Stricter error handling in material management:

• The add_material method in Project now raises a ValueError if the material is already present in the collection, instead of printing a warning.
• The remove_material method in Project raises a ValueError if the material is used by any model and an IndexError if the index is out of range, instead of printing errors.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.54%. Comparing base (dd3cb93) to head (084d9d6).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #411      +/-   ##
===========================================
+ Coverage    94.52%   94.54%   +0.01%     
===========================================
  Files           54       54              
  Lines         5516     5517       +1     
===========================================
+ Hits          5214     5216       +2     
+ Misses         302      301       -1     

Flag	Coverage Δ
integration	38.31% <14.28%> (-0.01%)	⬇️
unittests	94.54% <100.00%> (+0.01%)	⬆️

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Files with missing lines	Coverage Δ
src/easyreflectometry/project.py	94.36% <100.00%> (+0.10%)	⬆️

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