Adjust return value for image load error in extract line & line path

[rlskoeser]

The _extract_line and _extract_path_line methods currently return a 3-tuple instead of a 2-tuple when there is an error loading the image file. This causes an error when the extract function is called by map or imap_unordered - it looks like a code error and it took me some digging to determine I was not setting the image path correctly for the build_binary_dataset method to be able to load the image files.

I've revised to return two values instead of three and added logging so that it will be easier to diagnose image problems; I wasn't sure if it should be a warning or an error, so kept it as a warning.

I added a unit test based on the existing tests for the build_binary_dataset method.