BUG: Added error reporting for scene file writing

The problem was that when the scene file (.mrml) saving
failed (e.g., because target was a read-only directory) then
no error was reported.

No error was reported for the sceneview thumbnail image
saving failure either.

From: Andras Lasso <lasso@queensu.ca>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23246 3bd1e089-480b-0410-8dfb-8563597acbee