a-donda

…or connections This one fixes #2. There is no need to set conn.err in this line, as it is [written over](https://github.com/vcabbage/go-tftp/blob/07909dfbde3c4e388a7e353351191fbb987ce5a5/conn.go#L729) in the lilne above if the problem persists.

When the connection is not very stable, waiting for client ACK can trigger an error and the conn.err field is initialized [here](https://github.com/vcabbage/go-tftp/blob/07909dfbde3c4e388a7e353351191fbb987ce5a5/conn.go#L737). Then ACK is eventually received after few retries...