Add maybePrintNodes for abstracting the node list

Since this bit of code (including the “when (isJust …)” is used in
multiple places, let's abstract it in a function that is used
consistently. One (bad?) side-effect is that all node lists are done
to stderr, including the ones from hbal where it was previously done
to stdout (is that fine?).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>