Give better error messages

When one rms well is mapped to two simulator wells:
```text
Could not import simulator names: Invalid mappings: Value error, A source_id can only have one cross-system mapping per target system
```
this is confusing to the users. In general we should improve the error message for validation errors due to the validation rules that we have.

This is somewhat lower priority for now because we can guard this in the GUI, so this error message is not reachable from GUI flow.

Start with the validation rules and simulator-name import path that produce the shown mapping error. Review how validation errors are assembled, then make the messages clearer for this case and other validation-rule failures; done means the reported errors are understandable outside the GUI flow.

Written by the indexing model from the issue text.