fastboot: Check that reboot to userspace succeeded.

After rebooting to userspace fastboot, the first command issued is
"update-super". If we wound up in the bootloader by accident (as
happened to a few users with a busted vbmeta_system), the resulting
error message will be very misleading ("update-super" will be an
unrecognized command).

Instead, this patch explicitly errors if we did not successfully boot
into fastbootd.

Bug: N/A
Test: fastboot flashall
Change-Id: I7d5bd64db540978bd0bae884f40ce8c3df48f856