init: add more logging around reboots

Particularly in the case of the device failing to reboot.  Some test
devices are showing that they've received the reboot message but
without rebooting.

Bug: 150863651
Test: prevent init from handling reboot and see a stacktrace
Test: reboot works normally
Change-Id: Ide001dadbb9e9cd235ea509066e6ae6664bb429b