Issue BlobStore calls before opening Writer
Issue BlobStore calls before opening Writer

This allows Jetty to propagate errors from the calls correctly.
Follow on to dba252bb996dd75bcdf2398716725e43c7aa4498.

timuralp/s3proxy,andrewgaul/s3proxy,andrewgaul/s3proxy,kahing/s3proxy,flandr/s3proxy,flandr/s3proxy,kahing/s3proxy,timuralp/s3proxy