WARNING expression

   Synopsis:
      Writes an 'warning' message to the 'main' window

   Notes:
      The message is also written to Axmud's logfiles (if logging is enabled).
      See also the help for DEBUG, ERROR, PRINT and WRITE.

   Examples:
      ! Get a number between 1 and 6
      LET number = int (rnd (6) + 1)

      ! The subroutine will never be called
      IF number = 0 THEN CALL terminate()
      END

      SUB STRING terminate ()
         WARNING "This subroutine should not have been called!"
         STOP
      END SUB
