go-sdk: panic on malformed S2-compressed poll response

[atharvalade]

DeserializeFetchMessagesResponse in Go SDK calls panic() when S2 decompression fails, instead of returning an error.

payloadSlice, err = s2.Decode(nil, payloadSlice)
if err != nil {
    panic("iggy: failed to decode s2 payload: " + err.Error())
}

A truncated or corrupt compressed message from the server crashes the entire Go client process. This is a DoS vector clients should never panic on bad server data.

We should return error instead of calling panic.