[cells.js] added debugger call into future start exception path

Change-Id: Ie13d76f436a7c698f545a3e38eb9864003ddf5c0
Reviewed-on: http://review.couchbase.org/41007
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Pavel Blagodov <stochmail@gmail.com>
Tested-by: Pavel Blagodov <stochmail@gmail.com>