Empty Reply ≣ Server Error

Treating empty replies from the server like server errors (which, for all intents and purposes, they basically are). Keep retrying (including in our server retry count).

Signed-off-by: Seth Battis <SethBattis@stmarksschool.org>
Merge branch 'refs/heads/dev-canvas-api' into dev-calendar-ics

Signed-off-by: Seth Battis <SethBattis@stmarksschool.org>

Conflicts:
	www/api/canvas-api.inc.php