Error out if required config is absent

There's no point to continue if Client ID and/or Client Secret are missing.

Author: twz123

main.go

@@ -206,6 +206,16 @@ func main() {
 		clientSecret = *clientSecretFlag
 	}
 
+	if clientID == "" {
+		fmt.Println("No Client ID specified.")
+		os.Exit(1)
+	}
+
+	if clientSecret == "" {
+		fmt.Println("No Client Secret specified.")
+		os.Exit(1)
+	}
+
 	launchBrowser(*openBrowser, oauthUrl, clientID)
 
 	reader := bufio.NewReader(os.Stdin)