[Feature request] support reject with message

[zWingz]

When I set up an ssh tunnel and access a non-existent port, I will receive an error with Connection refused

But I can't throw this message by ssh2.

connection.on('tcpip', (accept, reject) => {
  reject('some message')
})

maybe it can be fixed in https://github.com/mscdex/ssh2/blob/master/lib/server.js#L548-L562