MAINT-6584: Rationalize custom KDU error/warning message classes.

Derive them both from a common base class that does the message logging,
instead of having each handler class log redundantly -- especially since the
put_text() override accepting const kdu_uint16* was simply streaming the
kdu_uint16 pointer to the log file, which would log the hex value of the
pointer.

Although we want a static instance of each of these handler classes, pull it
out rather than nesting the instance within the class itself.