Added close related exceptions

Author: WyriHaximus

src/ClosedException.php

@@ -0,0 +1,11 @@
+<?php declare(strict_types=1);
+
+namespace WyriHaximus\React\Parallel;
+
+final class ClosedException extends \Exception
+{
+    public static function create(): self
+    {
+        return new self('Pool is closed and won\'t run your Closure');
+    }
+}

src/ClosingException.php

@@ -0,0 +1,7 @@
+<?php declare(strict_types=1);
+
+namespace WyriHaximus\React\Parallel;
+
+final class ClosingException extends \Exception
+{
+}