Lower logging level in grading condition from exception to info

addresses part of LMS-2907

lms/djangoapps/courseware/grades.py

@@ -259,7 +259,7 @@ def _grade(student, request, course, keep_raw_scores):
             if graded_total.possible > 0:
                 format_scores.append(graded_total)
             else:
-                log.exception("Unable to grade a section with a total possible score of zero. " +
+                log.info("Unable to grade a section with a total possible score of zero. " +
                               str(section_descriptor.location))
 
         totaled_scores[section_format] = format_scores