Please provide more details in onSocketError

[alf]

Currently it's impossible to identify the cause of connection errors since this information is swallowed in https://github.com/rethinkdb/horizon/blob/next/client/src/index.js#L50. We do get an error status from https://github.com/rethinkdb/horizon/blob/next/client/src/socket.js#L147, but since there are no details about the reason this does not help us much.

Our problem is related to #248. We want to write robust code and give proper error messages to our users. Currently we see no way to accomplish this since the information is being dumped to the console instead of an endpoint we can subscribe.