The NumericException(event) function returns the exception string associated with event.

MyHandler := proc( f, ops, defVal )
    if NumericEventLocation = 'MyProc' then
        error NumericException( 'division_by_zero' );
      else
        NumericStatus( 'division_by_zero' = false );
        defVal;
    end if;
end proc:

MyProc := proc(a,b) a/b; end proc:

Error, (in unknown) numeric exception: division by zero