- is_default is required on the add_service_email_reply_to_request schema

- Added check that the service exists for the POST reply-to methods.
- Added tests

app/dao/service_email_reply_to_dao.py

@@ -75,10 +75,10 @@ def _get_existing_default(service_id):
         if len(old_default) == 1:
             return old_default[0]
         else:
-            # is this check necessary
-            raise InvalidRequest(
+            raise Exception(
                 "There should only be one default reply to email for each service. Service {} has {}".format(
                     service_id, len(old_default)))
+    return None
 
 
 def _reset_old_default_to_false(old_default):