ENH: Replace asserts by error handling in vtkMRMLScene

Problem: Essential checks were performed in vtkMRMLScene using asserts. These "work" in debug builds (makes the application crash) but asswerts are ignored in release builds.
Therefore, lack of checks caused crashes or silent errors in release builds.

Solution: Asserts were replaced by warning logs (instead of silently ignoring the errors) and/or returning with failure (instead of crashing).

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24828 3bd1e089-480b-0410-8dfb-8563597acbee