Fix error message for split-brain

In a split-brain situation, if a user tries to run a command (other than SET/SHOW), it will fail with the following error :

ERROR 1047 (08S01): Unknown command

It makes little sense. However, during code inspection I found that
actually a better error message was used wrongly with my_error(),
which eventually ignore it. (fix : use my_message() instead)

..
my_error(ER_UNKNOWN_COM_ERROR, MYF(0),
"WSREP has not yet prepared node for application use");
..