Handle THOLD alerts with no image/jpg or otherwise missing attachment

* Handle multipart message with no image/jpg subpart
* Remove DEBUG print statements from get_image

thold-slack.py

@@ -213,11 +213,12 @@ def get_image(message):
         # get the FIRST image JPEG version only
         image_parts = [part for part in
                        typed_subpart_iterator(message, 'image', 'jpg')]
-        image_part = image_parts[0]
+        image_part = "" if len(image_parts) is 0 else image_parts[0]
         return return_image(image_part)
 
     else:
-        attachment = message.get_payload()[1]
+        payload = message.get_payload()
+        attachment = "" if len(payload) is 0 else payload[1]
         return return_image(attachment)