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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CHANGELOG.md

@@ -9,8 +9,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added ###
 
+* `apiutils.EntityMergeConflictError` class for unresesolvable merge conflicts
+  when merging two entities
+
 ### Changed ###
 
+* `apiutils.merge_entities` now raises an `EntityMergeConflictError` in case of
+  unresolvable merge conflicts.
+
 ### Deprecated ###
 
 ### Removed ###