Update epersonregistrations.md: add description of status code 400 in PATCH

Author: saschaszott

epersonregistrations.md

@@ -173,7 +173,8 @@ We can have the following cases:
 Status codes:
 
 * 204 Created - if the operation succeed with a new token generated, and e new mail sent to the `email` provided in the request
-* 401 Unauthorized - if registration is disabled the token provided is not valid or absent
+* 400 Bad Request - if the `id` or `token` provided in the URL is absent or negative / blank
+* 401 Unauthorized - if registration is disabled or the token provided is not valid
 * 422 Unprocessable Entity - if the email address was omitted or the operation is not valid
 
 ## Create new EPerson registration