Fix/ichuang/instructor dashboard grades for assignment

Dumping of grades for a specific assignment is an important feature for residential use of edx-platform.

The instructor dashboard (what has been renamed to become the "legacy" version) has a bug in the code which provides this feature.  If a student's grades are missing entries, the dump will hit an exception due to an IndexError.  Instead, the student missing grades for that assignment should be skipped.  This PR makes that fix.