MDEV-29707 : Incorrect/bad errno on enabling wsrep_on after setting dummy wsrep_provider on non-Galera build

Fix error message to contain correct errno. This commit was
tested interactively because mtr will notice if you provide
wrong wsrep_provider in config and you may not change
wsrep_provider dynamically.

Author: Jan Lindström

sql/wsrep_var.cc

@@ -120,8 +120,7 @@ bool wsrep_on_update (sys_var *self, THD* thd, enum_var_type var_type)
 
       if (wsrep_init())
       {
-        my_error(ER_CANT_OPEN_LIBRARY, MYF(0), tmp, my_error, "wsrep_init failed");
-        //rcode= true;
+        my_error(ER_CANT_OPEN_LIBRARY, MYF(0), tmp, errno, "wsrep_init failed");
         saved_wsrep_on= false;
       }