Change error reporting mechanism so that condition types have an

Change error reporting mechanism so that condition types have an
explicit reporting procedure.  This procedure receives the condition
and an output port as arguments and can do anything it likes.

New procedure `condition/internal?' is used to filter out complicated
conditions.  If handlers always ignore conditions satisfying this
predicate they will never have to do anything hairy.

`format-error-message' now takes a port as a third argument.