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.

If you initialized the flag to False, you would only need to modify it when there is an error (which means that you wouldn't need lines 399 and 404), and there wouldn't be the risk that somebody modifying the method later forgets to set the flag to false in another non-error branch.