feature #2 Introduce new StatementIdAlreadyExistsException (Lctrs)

This PR was squashed before being merged into the 0.2.x-dev branch (closes #2).

Discussion
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Introduce new StatementIdAlreadyExistsException

Prepare moving the check for existing statement to the storage layer.

Commits
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ea8ae95 Introduce new StatementIdAlreadyExistsException

Author: xabbuh

CHANGELOG.md

@@ -4,6 +4,7 @@ CHANGELOG
 0.2.0
 -----
 
+* Added `StatementIdAlreadyExistsException`.
 * Added `UnsupportedStatementVersionException`.
 
 0.1.1

src/StatementIdAlreadyExistsException.php

@@ -0,0 +1,25 @@
+<?php
+
+/*
+ * This file is part of the xAPI package.
+ *
+ * (c) Christian Flothmann <[email protected]>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Xabbuh\XApi\Common\Exception;
+
+/**
+ * Statement id already exists exception.
+ *
+ * @author Jérôme Parmentier <[email protected]>
+ */
+class StatementIdAlreadyExistsException extends XApiException
+{
+    public function __construct($statementId, \Exception $previous = null)
+    {
+        parent::__construct(sprintf('A statement with ID "%s" already exists.', $statementId), 0, $previous);
+    }
+}