Add error handling in fathomnet-generate for failing image downloads

[kevinsbarnard]

If an image fails to download, the fathomnet-generate script will crash. Add error handling to log an error/warning and proceed with other images. These images should be excluded from the dataset.

In the case that the image should be re-fetched and added to the dataset once the link is working, re-running the same fathomnet-generate command will not re-fetch already-downloaded images, and instead only download the working images.