Return 404 when no chapters are found (#161)

server.py

@@ -579,6 +579,8 @@ def suggest_chapter(lectureid):
 @app.route('/chapters/<int:lectureid>')
 def chapters(lectureid):
 	chapters = query("SELECT * FROM chapters WHERE lecture_id = ? and visible ORDER BY time DESC", lectureid)
+	if not chapters:
+		return 'No chapters found', 404
 	last = None
 	for c in chapters:
 		c['start'] = c['time']