... But if you really want to go with custom error handling function, it is enough if you copy Example of an Error Handling Function from Oracle Doc here : Documentation and use it as described in one in your earlier post in this theread. In Oracle doc it's explain in details how to adjust it and where so that it will serve your need.

When I used it basicaly as it is, with only 1 small adjustment for I was not patient enough to create my own CUSTOM error table:

        -- If it's a constraint violation like
        --
        --   -) ORA-00001: unique constraint violated
        --   -) ORA-02091: transaction rolled back (-> can hide a deferred constraint)
        --   -) ORA-02290: check constraint violated
        --   -) ORA-02291: integrity constraint violated - parent key not found
        --   -) ORA-02292: integrity constraint violated - child record found
        --
        -- try to get a friendly error message from our constraint lookup configuration.
        -- If the constraint in our lookup table is not found, fallback to
        -- the original ORA error message.
        if p_error.ora_sqlcode in (-1,-2091, -2290, -2291, -2292) then
            l_constraint_name := apex_error.extract_constraint_name (
                                     p_error => p_error );
        
            begin
                select 'Some ridiculus message becasue I do not have custome message table'
                  into l_result.message
                  from dual;
            exception when no_data_found then null; -- not every constraint has to be in our lookup table
            end;
        end if;

I get this when violating FK:

I guess it is worth to try each those 2 and use the one that best fits your needs

Interactive Grid.

Solution that I used to find duplicates among newly entered records for the MGR field I wanted to keep unique.

Right click on the MGR column and select Create Validation.

1. Type = Function Body (returning Boolean)
2. Language = PL/SQL
3. PL/SQL Function Body:

declare ln_count number :=0;
begin
	if :APEX$ROW_STATUS = 'C' then
		select count(*) into ln_count
		  from APEX_COLLECTIONS
		 where collection_name = 'EMPLOYEES'
           and c001 = :MGR;
        if ln_count = 0 then
			APEX_COLLECTION.ADD_MEMBER ('EMPLOYEES',p_c001 => :MGR);
		else return false;
		end if;
	end if;
end;

I must guarantee my collection exists. To do that I am going to create Dynamic Action with PL/SQL code:

begin
APEX_COLLECTION.CREATE_OR_TRUNCATE_COLLECTION(p_collection_name => 'EMPLOYEES');
end;

Dynamic Action is going to be fired by Event = Save [Interactive Grid] with Fire on Initialization = TRUE (I need it even before the first Save action, just after my page gets loaded).

Reference:
APEX Collection.

You just need to remember that what this code actually does is with every selection change it loops through all the records and:

1. IF see selected it set collection  = service charge. (even if collection was modified by user it will still be replaced with service charge with next selection change detected)
2. It reverts all the changes for the remaining records. (it does not only revert collecion charge to its initial value, but it reverts all eddited fields of all unselected records)

I am not saying it does not do what you wanted, but before you apply it, please analzie it carefully and adjuste it to your needs.