Previously, the function would just return a presumed filename. Now that
it actually checks s3, if the file doesn't exist it'll raise an
exception. By default that's a StopIteration at the end of the bucket
iterator, which isn't ideal as this will get supressed if the function
is called within a generator loop further up or anything.
There are a couple of places where we expect the file may not exist, so
we define a custom exception to rescue specifically here. I did consider
subclassing boto's ClientError, but this wasn't straightforward as the
constructor expects to know the operation that failed, which for me is a
signal that it's not an appropriate (re-)use of the class.
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