Watch doesn't handle error cases

[bra-fsn]

I do a watch for a key prefix with client.watch_prefix('prefix'). The server is an etcd gateway (the TCP proxy).
It turns out etcd3-gateway doesn't handle at least two error cases:

1. when I restart the (upstream) etcd nodes, the etcd gateway closes the JSON streaming "array" and ends the connection with: {"error":{"grpc_code":14,"http_code":503,"message":"rpc error: code = Unavailable desc = transport is closing","http_status":"Service Unavailable"}}
2. when I cut the etcd gateway's TCP connections to the upstream servers it just closes the HTTP connection

Currently etcd3-gateway handles none of these, making the watch call hang forever.