Blender Git Loki

Fix T92318: adding layers (UVs, ...) doesn't notify about limit

When adding certain customdata layers (namely UVs, vertex colors and
sculpt vertex colors), the user does not get notified the specific limit
has been hit (blender just silently does nothing).

Now inform the user [decided to not do this in poll() since it could get
messy once operators are extended to operate on all selected objects, so
left this as a visible error in execute() -- or from python].

Maniphest Tasks: T92318

Differential Revision: https://developer.blender.org/D13147