MB-7656 Rename error constant for consistency

For consistency with the other error values in the enum, add
_ERROR_ to its name so that COUCHSTORE_REDUCTION_TOO_LARGE
becomes COUCHSTORE_ERROR_REDUCTION_TOO_LARGE.

Change-Id: I6e07e903256e09749c034c66e0d3e768ce17f498
Reviewed-on: http://review.couchbase.org/26670
Reviewed-by: Fulu Li <fulu@couchbase.com>
Tested-by: Aaron Miller <apage43@ninjawhale.com>
Reviewed-by: Aaron Miller <apage43@ninjawhale.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>