bunch of bugs when external_lock() fails on unlock:
* mi_lock_database() used mi_mark_crashed() under share->intern_lock,
  but mi_mark_crashed() itself locks this mutex.
* handler::close() required table to be unlocked, but failed
  external_lock didn't count as unlock
* mysql_unlock_tables() ignored all unlock errors, but they still set
  the error status in stmt_da.