MB-10451 Provide more verbose error message for reducer errors

View engine used to provide better error messages related to
reducer failure in Couchbase 2.5. Reducer error messages during the
index build/update are currently logged into the couchdb log file.
But query response contains only a basic error message. Instead of
providing an error message "reducer failure", descriptive
error message that comes from the index builder/updater needs to be
propogated to query response.

Eg.
"Reducer: Error building index for view `test2`, reason: Value is not a number
(key \"Abbey_2897309\")"

Change-Id: I9c79d2593f251c46f184f72ec3f3d09b05e1df8f
Reviewed-on: http://review.couchbase.org/34487
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>