Fix for undefined property error

Depending on your php version you get an _undefined property: stdClass::$error in .../alfredtweet.php on line 366_ during authentication process when there is *no* error!
Wrapping property accessor with isset() fixes the problem.

There are multiple occurrences in alfredtweet.php with missing isset(), which this changes is taking care of.

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Occurred on 
OS X 10.7.5
PHP 5.3.26 with Suhosin-Patch (cli) (built: Jul  7 2013 18:22:47) 
Copyright (c) 1997-2013 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2013 Zend Technologies