Update twitterfetcher.php

fix to handle not existing and not authorized accounts.

    {
        request: "/1/statuses/user_timeline.json?screen_name=NotauthorizedAccount",
        error: "Not authorized"
    }

or

    {
        errors: [
            {
                message: "Sorry, that page does not exist",
                code: 34
            }
        ]
    }
Merge pull request #5 from ramirors/master

fix to handle not existing and not authorized accounts.