Fix crash when trying to send to an unreachable client

i've seen this appearing pretty frequently nowadays on the server i'm hosting:

[2023-12-06 14:47:46] I_Error(): SOCK_Send, error sending to node 3 ([REDACTED]:51216) #65: No route to host

the error No route to host maps to the EHOSTUNREACH error code, which seems to indicate some sort of routing problem to the client. adding it to the list of ignored error codes in SOCK_Send fixes it.