tools: fit_image: Use fit_image_get_data_and_size for getting offset/size

This is very similar to fit_image_get_data but has the benefit of working
on FIT images with external data unlike fit_image_get_data. This is
useful for extracting sub-images from type of FIT image as this would
previously just silently fail. Add an error message also so if this
still fails it is easier to find out why.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>