docked-manager: Check whether docked mode can be enabled

For some reason, docked mode sometimes gets enabled on its own (#421). The reason why that happens is still unknown.

When that happens while the user has no dock around and no way to easily ssh into the phone, the only thing that they can do in order to make the phone fully functional again is to reboot it - otherwise there's no OSK available and windows don't get maximized. Therefore, let's log an error and restart the shell when that happens in hopes that logs will provide better insight about the issue (with recovering the user session as a positive side effect).