[Tizen] Fix widget namespace value validation.

According to widget W3C spec (http://www.w3.org/TR/widgets/#namespace), the namespace cannot be absent or invalid.
If namespace is absent or wrong, the widget should be treated as invalid package and installation should be failed.

BUG=XWALK-2978
Merge pull request #2655 from yejingfu/fix-widget

[Tizen] Fix widget namespace value validation.