Update to show if there is no enrollments or error in calling Canvas

Summary of Changes:

(1) check return from Canvas and set flag
      (a) Canvas return list of enrollments  (existing); set flag to false
      (b) Canvas returns empty list of enrollments  (new); set flag to false
      (c)  Canvas return an error, but canvas_utils.py returns None (existing); set flag to true
(2)  pass flag to templates;  if flag is false, display counts, if flag is true, display error message to user
(3)  update code based on code review feedback

Note:  This pull request does not include the exact UI text to be displayed back to user.  Roderick will be making those changes.