error: Fix examples in error.h's big comment

Mark a bad example more clearly.  Fix the error_propagate_prepend()
example.  Add a missing declaration and a second error pileup example.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <20200707160613.848843-2-armbru@redhat.com>
1 file changed