Improve exception logging for system backup/restore

Make sure to log stack traces of unexpected exceptions
to allow us to better diagnose issues such as b/190023280

Change-Id: I29a32329f52b8346570148ed5abc34f3149aa0f9

Test: make
Bug: 190023280
Change-Id: Idd4e4968fe4c642b463149f98a7669a8733b37d7