Explicit error message when setting active a Viewport that is already active.
Explicit error message when setting active a Viewport that is already active.

Co-authored-by: Hugo Locurcio <1720facb761ad1b49c72c19a004605f1a7d73bc4@hugo.pro>
(cherry picked from commit e3be0913d73bdc1ff45acef30209df519c0eff2a)

ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot