Fixed fatal error in paypal gateway

This line (542) should use `$response` instead of the not existing `$result`.

The fatal error only occurs when debugging is enabled and the communication with Paypal API failed for some reason. But in this case, it results in a fatal php error: Call to a member function get_error_message() on a non-object

btw: the actual http error message that it should have written to the log in my case was "Unknown SSL protocol error in connection to www.paypal.com:443" - can't tell why this happened. probably just a network hickup since the next request went though... so it's a rather rare case, but still a fatal php error.