retryq: Use mc_EXPACKET

This uses mc_EXPACKET to preserve the original error information of the
packet.

Change-Id: Id89a2449c14daaa3879df5abc91e80d916412069
Reviewed-on: http://review.couchbase.org/39754
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>