BUG: unify how add/remove handle error cases

Remove the error print out when removing a view node that
is not in the list, since when adding a view node that is
already in the list no error is printed. The code recovers
gracefully, no need to post an error.

Issue #3216



git-svn-id: http://svn.slicer.org/Slicer4/trunk@23085 3bd1e089-480b-0410-8dfb-8563597acbee