[Ephemeral]: MB-23734: Handle DCP backfill failures

[Ephemeral]: MB-23734: Handle DCP backfill failures

When a DCP backfill task is run there could be failures. We must handle
the failures gracefully.

This commit handles any failures in DCP backfill in Ephemeral buckets.
Upon a failure we close the stream and the DCP client can retry at
a later time.

Change-Id: I2aeffb9baf7d5a8ac367b129470741af7806e710
Reviewed-on: http://review.couchbase.org/77215
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>