Change error code for Galera unkillable threads

See also: https://github.com/codership/galera/issues/447

Galera can mark user threads as high priority and so they can't be killed, the check is done in the same condition that checks the privileges for killing threads/queries and so the error message is the same, which in case of galera unkillable thread is misleading "You are not the owner of the thread .... " even if you are the owner or a fully privileged user.