Improve handling of Riak disconnection errors on object downloads

Change the behavior when a Riak connection is closed during an object
downloading. There is existing retry logic to handle the case when
n_val=1 block requests fail with a notfound or timeout error. It is
desirable to also retry requests in the case of a transient connection
failure, especially in the case of the download of a sizable
object. Allowing the disconnect error to trigger the retry logic
provides time for the automatic reconnection process to work and
provides a mechanism for in-progress downloads to continue and
complete successfully.
Merge branch 'bugfix/retry-block-fetch-on-disconnect' into release/1.4