Add an 'invalid password' error

usersystem.capnp

@@ -36,9 +36,13 @@ struct UserSystem
             enum AddUserError {
                 alreadyExists @0;
                 # An user with that username already exists
+
                 usernameInvalid @1;
                 # The provided username is unusable, e.g. contains invalid characters,
                 # is too long or too short.
+
+                passwordInvalid @2;
+                # The provided password is unusable, e.g. it's of zero length
             }
             error @0 :AddUserError;
         }