bad error message if png output dir doesn't exist

[doutriaux1]

code seems to keep going even though it can't create the png file, we should probably error exit

ERROR: In /home/doutriaux1/build_15_04/build/VTK/IO/Image/vtkPNGWriter.cxx, line 260
vtkPNGWriter (0x4b8f630): Unable to open file pngs/1870-10.png