Related docs: manual doc picker lacks localization of error messages

org.onehippo.forge.relateddocs.editor.DocumentPickerDialog is doing:

error("Failed to determine validity of selection");
error("No node selected");
error("You cannot add the same document as the related document");

The messages are shown directly in UI in English, regardless of the language selected on login.

Also HIPPLUG-1599 needs a 4th message.