Proposal: reject connect() on authentication error

Hi, I have a use case where authentication is required:
```js
const logger = new FluentClient("output", {
socket: {
host: "fluentd.local",
port: 24224,
timeout: 3000,
disableReconnect: true,
tls: {
ca: require('fs').readFileSync('./fluentd.crt'),
},
},
security: {
clientHostname: "client.local",
sharedKey: "incorrect",
username: "ooo",
password: "xxx"
},
});

try {
await logger.connect();
} catch (err) {
/* do something with err */
}
```
When security is not correctly configured, `connect()` still resolves.

Though the error could be detected by `logger.socketOn('error')`, it's not synchronous with `connect()` and some mechanism to wait for the error (or to ensure no error thrown) is required.

If `connect()` can reject on error, it would be more easy to determine whether the client is authenticated successfully.