gs-details-page: Don't proceed to stage2 on cancellation

We don't want to proceed to stage2, if the details page request has been cancelled. This is most probably caused by the user switching back to the Overview page, while in "Loading app details..." page.

Proceeding to stage2 causes side effects like setting the page state to GS_DETAILS_PAGE_STATE_READY, which causes the app details page to be displayed with only partial filled app info, until updated fully a few seconds later.