[Feature] 8233 fix error messages from different objects

[``Fix error message layout when multiple faulty objects are involved``](https://www.openproject.org/work_packages/8233) (see [the comments](https://www.openproject.org/work_packages/8233#note-28))
This can be fixed in Rails 4 where the ``flush`` parameter was introduced. But if persist on solving this another way I can give it a try.