[Beta][Tizen] Fix Glib assertion due to appcore_context.

This patch get rid of following log:
[ERROR:browser_main_loop.cc(228)] GLib: g_source_destroy: assertion `source != NULL' failed

AppcoreContext::Initialize() fails when we launch xwalk in the shell. e.g. > xwalk
It is because appcore_init() can success only if registered application (e.g.
web applications) is launched.

This patch handles the exception that AppcoreContext::Initialize() fails.

(cherry picked from commit c5c38db9958f74629ffddd17f46a75853fdaaec7)

BUG=https://crosswalk-project.org/jira/browse/XWALK-46