Improved the warning for vtkInteractorStyleSwitchBase

Only warn if something attempts to get the Interactor, and the base
class is being used. The interactors are not always needed, and so we
may want to add some way to prevent this message from being emitted.

Change-Id: I4a19144db6e554379a676bca2111ad051a29c3b0
Merge topic 'interaction-improve-warning' into master

59977b62 Improved the warning for vtkInteractorStyleSwitchBase