pf: improve source node error handling

Functions manipulating source nodes can fail due to various reasons like
memory allocation errors, hitting configured limits or lack of
redirection targets. Ensure those errors are properly caught and
propagated in the code. Increase the error counters not only when
parsing the main ruleset but the NAT ruleset too.

Cherry-picked from development of D39880

Reviewed by: kp
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D39940