match original API

This is probably just as BC breaking as now we are throwing an Exception
where before a PHP fatal error was being thrown. But it does match the API
correctly, and I admittedly am the one who demands that is the
documentation that makes up the API, not the data returned.

framework/Crypt/lib/Horde/Crypt/Pgp.php

@@ -1483,4 +1483,12 @@ public function getPublicKeyFromPrivateKey($data)
             : $result->output;
     }
 
+    /**
+     * @deprecated
+     */
+    public function generateRevocation($key, $email, $passphrase)
+    {
+        throw new Horde_Crypt_Exception('Not supported');
+    }
+
 }