pvbatch warnings if directory missing are confusing

If the directory that pvbatch is trying to write to is missing, the warning message is way confusing. A user just tried to debug this:

^[[0m^[[31m(   7.161s) [main thread     ]     vtkImageWriter.cxx:509    ERR| vtkPNGWriter (0xc89d520): Ran out of disk space; deleting file(s) already written^[[0m
^[[0m^[[31m(   7.410s) [main thread     ]       vtkPNGWriter.cxx:253    ERR| vtkPNGWriter (0xfb61b70): Unable to open file ../output/quench_avg_eqps.15.png^[[0m
^[[0m^[[31m(   7.410s) [main thread     ]     vtkImageWriter.cxx:509    ERR| vtkPNGWriter (0xfb61b70): Ran out of disk space; deleting file(s) already written^[[0m
^[[0m^[[31m(   7.843s) [main thread     ]       vtkPNGWriter.cxx:253    ERR| vtkPNGWriter (0xf843820): Unable to open file ../output/quench_avg_eqps.16.png^[[0m

Please change the code to TEST if the directory exists, and give a warning appropriately. If the file can't be written, then say something like "File can't be written. Possible issues are permission issues or the disk is full".

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