bootctl: add is-slot-marked-successful command.

This makes it easy to call the newly added isSlotMarkedSuccessful()
boot_control HAL method added in the CL at

 https://android-review.googlesource.com/#/c/185841/

This is useful for e.g. test suites.

Also improve error reporting for is-slot-bootable command.

Change-Id: I603e07d8310fc1de88114dadbaa1622a76289afb
Test: tested on edison (Brillo).
1 file changed