Make sure that we don't mutate the upload queue, until we're sure that
we will complete all the changse. Otherwise, we could bail out after
removing some files from upload_queue.latest_files, but not all.
This is purely theoretical: there were only two ? calls in the
function, and neither of them should actually return an error:
1. `RelativePath::from_local_path` only returns error if the argument
path is not in the base directory. I.e. in this case, if the argument
path was outside the timeline directory. Shouldn't happen.
2. latest_metadata.to_bytes() only returns an error if the serde
serialization fails. It really shouldn't fail.
I considered turning those into panics instead, but as long as the
function as whole can return an Err, the callers need to be prepared
for that anyway, so there's little difference. Nevertheless, I
refactored the code a little to make it atomic even one of those
can't-happen errors happen after all. And I used a closure to make it
harder to introduce new dangerous ?-operators in the future.
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