Application Express :: How To Correctly Trap PL/SQL Errors

I have a page process that calls back end PL/SQL. I have a page item that is populated by said PL/SQL procedure, P35_PROCESSING_MSG. As you know the Process allows me to specify a message for Success and another for Failure. My desire is that, if there is a back end failure, the error will display in the dialog area, using the built-in "Process Error Message" functionality. Instead, after execution, the page item still has its default value and the following, ugly, error displays in the tabular report region.

report error:
ORA-20001: Error fetching column value: ORA-01403: no data found
ORA-06510: PL/SQL: unhandled user-defined exception

I just don't get why it displays here instead of in the usual failure area. Moreover, I don't understand why the error is "*+unhandled+* user-defined exception" when, as you will see, I have handled it.

I have a page process to perform custom MRD for a tabular form. (I don't think this is really germane to the more generic issue but, I bring it up as an explanation for the following code sample). I am deliberately causing a DIVIDE BY ZERO error to test the error handling form the back end.

PROCEDURE PROCESS_MARGIN_CALL_DELETES( as_StatusMsg OUT VARCHAR2 )  
IS
lb_InnerErrorOccured    BOOLEAN := FALSE;
ln_DeleteTargetCnt      NUMBER := 0;
ln_DeleteTargetRow      NUMBER := 0;
BEGIN
[code].......