fix: catch more sample merging edge cases

[alexander-held]

Catch a few more edge cases: sample to be merged does not exist (providing a better error message), new sample has same name as existing unmerged sample leading to duplicate sample names (e.g. ["S", "B"] updated with {"S": ["B"]} resulting in ["S", "S"].

to-do: tests

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.81%. Comparing base (0252dee) to head (8517e12).

Files with missing lines	Patch %	Lines
src/cabinetry/model_utils.py	0.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##            master     #560      +/-   ##
===========================================
- Coverage   100.00%   99.81%   -0.19%     
===========================================
  Files           22       22              
  Lines         2132     2136       +4     
  Branches       300      302       +2     
===========================================
  Hits          2132     2132              
- Misses           0        2       +2     
- Partials         0        2       +2     

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