[Tizen] Fix of backendlib - pkg_plugin_get_app_detail_info_from_package should validate widget path

If pkgmgr-server passes path that is not valid widget path,
this will lead to crash of pkgmgr-server. Function
pkg_plugin_get_app_detail_info_from_package of backendlib
should check if valid package data can be received from
given path.

BUG=XWALK-2683
I think what made this more confusing is that chromium-cpp.appspot.com talks about trunk, while we're currently tracking M38. This isn't a problem for Tizen because its GCC 4.8 should be recent enough to allow those features, but the change on our side to start allowing C++11 hasn't been widely announced (if at all).