Ensure we don't read a MariaDB 5.3 errmsg.sys file or new errmsg.sys file with holes for not used error messages

sql/derror.cc:
  Ensure we don't read a MariaDB 5.3 errmsg.sys file with moved error messages or a new errmsg.sys file with holes for not used error messages
  If error message file didn't exist and we have not read one in the past, don't continue.
  Give better error message if the errmsg.sys header has changed.