Improve exception handling

Catch exceptions and continue syncing items
in that shard but do NOT update the worker
bounds. This will result in redundant work
but prevents an error from stopping
the syncing of objects 'after' that object
in the same shard.

Signed-off-by: Joe Buck <jbbuck@gmail.com>
Reviewd-by: Josh Durgin <josh.durgin@inktank.com>