Missing Error Message when posting with viewable for other but without selecting a group

[Stephan Doerfel]

In the posting dialogue, when viewable for "other" is selected, and the user does not select a group, then - when clicking save - one is returned to the dialogue without explanation.

This is particularly confusing as it is easy to miss the group since the relevant-for field is also there, also containing groups. The relevant-for field is there all the time, while the "viewable-for" other field only appears after selecting "other". Thus fast users might click other and then select a group from the relevant-for field.

Related issues are #917 and #2212