Add more logging to fs_mgr_remount_userdata_into_checkpoint

Sometimes unmounting userdata fails, this should help in debugging.
Ideally this information should be persisted somewhere so that we can
add it to the bugreport, but so far I don't have solution better than
writing it to a file in /metadata partition.

Test: adb reboot userspace
Bug: 135984674
Bug: 143970043
Change-Id: I92489600bf31f7f47b8b87de7cd4a882be21910e