Magnifier: demote exceptions reading focus/caret position

It's possible that the DBus call goes in timeout (which is bad,
but unavoidable, given that atspi is synchronous) or fails
because the component has been removed (race condition). Those
errors are not dependent on gnome-shell, but on faulty applications
(mostly).
If they happen, log a debug message and continue.

https://bugzilla.gnome.org/show_bug.cgi?id=724305