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;)
I think we should move this to the representer. Model should contaion only properties that can be set, representing the resource in different states should be the responsibility of representer.