Return an error when the message pipe step throws an exception.
Return an error when the message pipe step throws an exception.

Change-Id: I50bbf6a8b2e6825a47d3b47d328e22d04f14fefc
Reviewed-on: http://review.northscale.com/2060
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>
Tested-by: Dustin Sallings <1c08efb9b3965701be9d700d9a6f481f1ffec3ea@spy.net>

membase/vbucketmigrator,zbase/vbucketmigrator,zbase/vbucketmigrator,zbase/vbucketmigrator,zbase/vbucketmigrator,membase/vbucketmigrator,membase/vbucketmigrator,membase/vbucketmigrator