F merge conflict error

Summary

Precondition for https://gitlab.indiscale.com/caosdb/customers/leibniz-zmt/management/-/issues/133. Introduces an EntityMergeConflictError class that's raised in case of merge conflicts in apiutils.merge_entities.

Focus

New error class in apiutils.

Test Environment

unittest should be sufficient.

Check List for the Author

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• All automated tests pass
• Reference related issues
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• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
• The test environment setup works and the intended behavior is reproducible in the test environment
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• Check: Are there specifications? Are they satisfied?

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