facebook-item: Bail out as soon as there is an error

If we failed to refresh the authorization tokens, then we should
immediately bail out instead of trying to retrieve the photo node from
the Facebook graph. This is because of two reasons:
 - the failure could have been due to the GCancellable being cancelled,
   in which case we should stop as soon as possible, or
 - there was an actual failure and we should not trample over the
   GError that was set

Fixes: https://bugzilla.gnome.org/727205
facebook-item: Handle failures gracefully without aborting

Each failure should be represented by setting the GError **, which
should be bubbled up to the caller. Use gotos and properly initialized
variables to clean up resources on both the error and success code
paths.

In case we failed to download an item, clean up the leftover local
file.

Fixes: https://bugzilla.gnome.org/727205
facebook-item: Plug a memory leak

Fixes: https://bugzilla.gnome.org/727205