Dos 3+ critical error handler - Handle error

        Entry:
          AH = 05h
          AL = extended error code (not zero)

        Return:
          CF clear
            ES:DI -> ASCIZ error message
            AL = ???
          CF set: use default error handler

        Note: called at start of COMMAND.COM's default critical error
              handler if installed by a user program, allowing partial or
              complete overriding of the default error message