pkg/blobserver/s3: Error when an s3 bucket contains non-blob files.
pkg/blobserver/s3: Error when an s3 bucket contains non-blob files.

Skipping these files as we currently do causes some clients to assume
they have enumerated all blobs (because fewer than 'limit' records were
returned), resulting in e.g. 'camtool sync' hanging.

Fixes #872

Change-Id: I1f6f6a5d784f76516c6e48d0bbf9df75abc65a4b

angelcabo/camlistore,stevearm/camlistore,marook/camlistore,mpl/camlistore,stevearm/camlistore,camlistore/camlistore,mpl/camlistore,mpl/camlistore,marook/camlistore,camlistore/camlistore,stevearm/camlistore,angelcabo/camlistore,marook/camlistore,marook/camlistore,angelcabo/camlistore,stevearm/camlistore,mpl/camlistore,stevearm/camlistore,camlistore/camlistore,angelcabo/camlistore,marook/camlistore,marook/camlistore,angelcabo/camlistore,camlistore/camlistore,camlistore/camlistore,stevearm/camlistore,angelcabo/camlistore,mpl/camlistore,mpl/camlistore,stevearm/camlistore,angelcabo/camlistore,angelcabo/camlistore,angelcabo/camlistore,camlistore/camlistore,stevearm/camlistore,camlistore/camlistore,camlistore/camlistore,mpl/camlistore,mpl/camlistore,mpl/camlistore,camlistore/camlistore,stevearm/camlistore,marook/camlistore,marook/camlistore