add connection manager and facade with implementation

src/Exceptions/ConnectionNotAvailableException.php

@@ -0,0 +1,18 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpMqtt\Client\Exceptions;
+
+/**
+ * Class ConnectionNotAvailableException
+ *
+ * @package PhpMqtt\Client\Exceptions
+ */
+class ConnectionNotAvailableException extends MQTTClientException
+{
+    public function __construct(string $name)
+    {
+        parent::__construct(sprintf('An MQTT connection with the name [%s] could not be found.', $name));
+    }
+}