Custom MONTITOR handling

Is it possible to "throw" custom messages from a subprocedure?

That is, I have a program called ShowCust.  It has a bit of code which looks
like this:

/Free
  openBalance = getOpenBalance( myCustNo );
  monitor:
    on-error ????;
      // handle customer not found
    on-error;
      // handle other errors
  end-mon;
/End-Free

getOpenBalance() is another procedure that accesses a file via SQL, and
calculates the current open balance.  I'd like it if it could throw an
exception message if the customer isn't found, the customer number is invalid,
etc.

Any ideas?

Thanks

-Doc

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