Improve error message in case of externalIds unique constraint error
Currently, externalIds[].identifier+serviceIdentifier
has a composite unique constraint. When a user tries to submit a form, where externalId.identifier+serviceIdentifier is already is use (either for an actor or for an item), the error message reads: "could not execute statement; SQL [n/a]; constraint [actor_external_ids_identifier_service_code_identifier_key]; nested exception is org.hibernate.exception.ConstraintViolationException: could not execute statement".
It would be good to return a more human readable error, since we cannot check for this case client-side before form submit.