Extend ON-ERROR to accept message codes

In our codebase, we use a lot of user-defined message codes in *ESCAPE messages for our exception handling. We combine these with RPG's MONITOR op-code to trap the errors and, if possible, handle them.

Identification of the last exception message code is not an issue. What is currently missing is an elegant mechanism to trap a specific subset of error codes.

Here is a simplified example of how our code might look currently:

------------------------
monitor;
someProcess();
on-error;
lastError = getLastException();
if lastError = 'USR0001';
// Remedial action
else;
// Send new escape message
endif;
endmon;
------------------------

Since ON-ERROR does not allow us to distinguish between different message codes we must capture all exception messages in a general ON-ERROR block. The problem here is that when we hit an exception we don't care about, we have the choice of either:
- leaving the handled message in the job log, adding a lot of noise; or
- removing the message, and losing the context of the original error.

If we could change ON-ERROR to only execute for particular message codes, we could leave other unexpected exceptions unhandled to percolate up the call stack.

If we could add handlers for specific message codes, RPG's native error handling would also more closely resemble exception handling features found in other languages e.g. Java, Python.

RFE 135394, to control job log output, would nicely complement this to bring rich exception handling directly into the RPG language.

An example could look like this:

------------------------
monitor;
someProcess();
on-error 'USR0001';
// Remedial action A
on-error 'USR0002' : 'USR0003';
// Remedial action B
on-error 'USR*';
// Remedial action C, for all other USRxxxx messages
endmon;

// Exceptions with message prefixes other than 'USR' would percolate up the call stack
------------------------

Another op-code could be used (ON-EXCEPTION?) if there is too much ambiguity with the existing ON-ERROR behaviour.