Added new exceptions for HTTP error codes.

src/HPCloud/Transport/UnprocessableEntityException.php

@@ -0,0 +1,12 @@
+<?php
+/**
+ * @file
+ */
+namespace HPCloud\Transport;
+/**
+ * Represents an HTTP 422 error.
+ *
+ * This often represents a case where a checksum or hash did not match
+ * the generated checksum on the remote end.
+ */
+class UnprocessableEntityException extends \HPCloud\Exception {}