F fix misleading error

Summary

For #127 (closed). This just improves the original error message and does not introduce additional error handling.

Focus

Since it was highly non-trivial to introduce error handling that could actually distinguish between (cf. docstring of new test) not being able to merge b1 and b2 because of unresolvable merge conflicts in c1 and c2 and only having to postpone the merge because of apparent merge conflicts that will be resolved down the line, we do not introduce new error handling. Instead, we point out that the error with missing referencing identifiables might be due to merge conflicts in the referencing entities.

Test Environment

Unit test

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