Be less verbose with handshake send errors

[nbrownus]

This dampens the log verbosity of messages like

level=error msg="Failed to send handshake message" error="sendto: no route to host" handshake="map[stage:1 style:ix_psk0]" initiatorIndex=1 localIndex=7 remoteIndex=0 udpAddr="[1]:51599" vpnAddrs="[2]"

Basically we log write errors at error level only if the list of remotes changes, this matches the success logging behavior and doesn't cost any additional memory.