Update exam_processing.py

hi suren, there is a bug in validate_sng_column (trying to return False for grade = "fa" and sng being blank but it doesn't return False)
i have been trying to get rid of it for an hour but just wouldn't work :'(

will continue tomorrow on this!
G
Merge pull request #6 from gaurav1780/patch-6

Update exam_processing.py