Address inconsistent-return-statements

Pylint warns about inconsistent return statements when some paths of a
function return None implicitly. Make all implicit returns either
explicit or raise a proper exception.

See: https://pagure.io/freeipa/issue/7758
Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>

ipatests/test_ipalib/test_frontend.py

@@ -206,6 +206,8 @@ class test_Command(ClassChecker):
             def __call__(self, _, value):
                 if value != self.name:
                     return _('must equal %r') % self.name
+                else:
+                    return None
 
         default_from = parameters.DefaultFrom(
                 lambda arg: arg,