fix: Error messages for relations

[mesemus]

Description

There are two issues with how errors in relation fields (e.g., references to vocabularies) are handled in RDM records.

• Error messages do not clearly indicate where the error occurred and what the error was.
• These errors behave significantly differently from other validation errors. Normally, validation is performed at the service level using the marshmallow library. This allows even invalid drafts to be saved: incorrect fields are omitted from the draft and validation messages are returned in the errors field of the response.

Relation errors, however, are handled later during record commit. As a result, they raise an exception, the draft is not saved, and the response is serialized differently.

This PR addresses only the first issue. The second is architectural in nature, and resolving it would require a larger change that cannot realistically be completed in time for v14.

Problem and solution

Previously, relation validation errors used generic messages such as "Invalid field value", which made debugging difficult, especially in complex record structures with nested relations.

This change replaces those generic messages with more specific ones that include:

• the invalid value
• the field path where the error occurred

This makes it significantly easier to identify and resolve relation validation issues during development and debugging.

Checklist

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