S3: Fix check of uuid file stored in bucket, which was not working.

S3: Fix check of uuid file stored in bucket, which was not working.

The check was broken in two ways.. First, nowhere did it error out when
checkUUIDFile found a different UUID already in the file. Instead,
it overwrote the uuid file.

And, checkUUIDFile's implementation was for some reason always failing with
a ConnectionClosed exception. Apparently something to do with using two
different runResourceT's and a response getting GCed inbetween. I'm pretty
sure that used to work, but changed to a more obviously correct
implementation.

This commit was sponsored by Peter Hogg on Patreon.