Improve streamrpc error messages

I came across a failing test during development, where a StreamRPC client was talking to a server that didn't support the protocol. The failure message was hard to decipher:

receive response: read unix @->/tmp/gitaly-639963007/gitaly.socket.743810280: read: connection reset by peer

After this change, the message would have been:

streamrpc client: receive handshake: read unix @->/tmp/gitaly-639963007/gitaly.socket.743810280: read: connection reset by peer

I think this gives a better hint where the problem is.

Part of gitlab-com/gl-infra&463 (closed)