Use full error messages for relations validation.

https://www.openproject.org/work_packages/13948

This PR should give sensible translated error messages when validating relations. Sadly they still appear at the top of the screen.

Also it should now respect the visibility of related work packages. So, as described in the ticket, if you make a relation and then the user loses the rights to view that work package then the relation will no long be visible (and there won't be an empty table row).

None of this is tested - sorry I've run out of time. Perhaps the reviewer @hschink could take over the responsibility;)
This approach leads to rather complex code on the client side. I think that if a related work package is not visible it shouldn't even appear in the list of relations (otherwise the user gains knowledge about a work package she is not allow to see). Thus, instead, I propose to filter [the list of relations in the work package representer](https://github.com/opf/openproject/blob/dev/lib/api/v3/work_packages/work_package_representer.rb#L183-L185).