the upload preview page has a file_id - this corresponds to the file in
the transient pdf uploads bucket. However, if the user already hit send
(and then navigated back) the file's no longer in that bcuket, it's been
moved to the regular letters-pdf bucket. So the s3 get request fails. To
avoid this, simply redirect to the notifications page if the file isn't
in the transient bucket. This is better for the user as it'll stop them
trying to submit it twice, and will provide more clarity on the status
of the notification too.
