this tries to catch the cases that:
- user tried to run the backup without first running "prepare backup..."
- the USB connection is a little flakey and returning bum data
We can only inspect the plaintext portions of the header, which contains
things like protocol version numbers, time stamps, and language codes.
We use the timestamp validation heavily because it's the only field that
isn't almost always going to be a 0 or 1 when correct, which are highly
likely to be reported anyways if there's a problem with the USB link.
This does assume the user sets the time correctly on the device
doing the backup, and on the Precursor itself.
