mod_s2s_keepalive: Fix identification of replies (error-replies included) Typo? This condition is supposed to filter out get and set (and anything else), not errors, which can still mean that there's connectivity, just no handling of XEP-0199.

This module writes out a traceback to `traceback.txt` in Prosodys data
directory (see `prosodyctl about`) when the signal `SIGUSR1` is
received. This is useful when debugging seemingly frozen instances in
case it is stuck in Lua code.