do not crash in zeta_client when server is unreachable

Hi,

I saw you worked on handling unavailable servers via zeta_ns in develop, but it still seems to be work in progress  (ie. it does not work yet).

Since I fixed a similar issue for my graphite client in folsomite I thought I would also make a patch for the zeta_client, it feels a bit like a workaround, but actually works just fine.

Feel free to use or ignore and pursue your original approach.

best regards,
  Fabian