Need graceful error for directories

[spficklin]

When I accidentally provided a path that was not a directory I got the following error:

granny -i cli -d /local/projects/granny/demo/cross_section_images/cross_section_tray/cross_section_demo_image.jpeg -a segmentation
Traceback (most recent call last):
  File "/home/ficklin/.local/bin/granny", line 8, in <module>
    sys.exit(run())
  File "/home/ficklin/.local/lib/python3.10/site-packages/Granny/GrannyBase.py", line 31, in run
    GrannyCLI(parser).run()
  File "/home/ficklin/.local/lib/python3.10/site-packages/Granny/Interfaces/UI/GrannyCLI.py", line 101, in run
    images = self.listImages(self.image_dir)
  File "/home/ficklin/.local/lib/python3.10/site-packages/Granny/Interfaces/UI/GrannyCLI.py", line 61, in listImages
    image_files: List[str] = os.listdir(image_dir)
NotADirectoryError: [Errno 20] Not a directory: '/local/projects/granny/demo/cross_section_images/cross_section_tray/cross_section_demo_image.jpeg'

We need a more graceful error message to report to the user