ensure id is valid in grade summary page

fixes CNVS-1869

test plan:
 - as a teacher, go to a student's grade summary page
 - in the url, replace the student's id with 'banana' or 'lqw'
   (e.g. /courses/1/grades/banana)
 - you should get a Page Not Found error page
 - navigate to /courses/:id/grades and the page should
   redirect you to gradebook

Change-Id: Ic7d2b3de463615957a8d4178544ab19b7e651f60
Reviewed-on: https://gerrit.instructure.com/29215
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>