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.

This last else condition will never happen with the way the the 'return_empty_list'  is being passed into fetch_list_page in canvas_utils.... The fetch_list_page  will return a list or an empty list and both those return conditions are handled in the blocks in line 395 or line 399. But it never retunrs None and so line 403 will never execute.