Recognise FileVault's rate-limiting

[galaxy4public]

If the user entered incorrect password multiple times during the installation, FileVault will lock the volume for 15 minutes. Currently, BootUnlock is unaware of this and will continue to attempt to unlock the volume. The worst is that even if you try to rollback by rebooting into a different volume where everything was working, the rate-limited volume won't be mounted. We need to recognise the rate-limit, possibly extracting the info for how long we need to wait, and provide some meaningful feedback to the user.