Always close file descriptors

Every error thrown leaked a netlink NETFILTER socket.

Test: as follows
    - built, flashed, booted
    - runtest frameworks-net passes
    - enabled tethering and tried to cause conntrack updates to sessions
      that had expired (which causes ErrnoExceptions to be thrown)
      while monitoring lsof for system_server's NETFILTER sockets

Bug: 32163131
Bug: 64976634
Bug: 110122306

Change-Id: Ib52b812e8434c27ad9f2596666400c13e03c2216