bucket.objects[object].exists? should rescue Errors::Forbidden on non-existent object.

bucket.objects[object].exists? should rescue Errors::Forbidden on non-existent object and result in false.

fixes #201


    > bucket.objects["test.js"].exists?
    => true
    > bucket.objects["test.js-nonexistent"].exists?
    AWS::S3::Errors::Forbidden: AWS::S3::Errors::Forbidden
    <stack trace>