Added exception for read-only objects.

src/HPCloud/Storage/ObjectStorage/ReadOnlyObjectException.php

@@ -0,0 +1,9 @@
+<?php
+/**
+ * @file
+ */
+namespace HPCloud\Storage\ObjectStorage;
+/**
+ * Thrown if an object that is read only is modified.
+ */
+class ReadOnlyObjectException extends \HPCloud\Exception {}