MB-7656 Add error for generic reducer errors

While this is not used/necessary for database reduce and
rereduce functions, it will be used by view reducers to
signal very view specific domain errors. Detailed information
regarding these errors will be set in the view_reducer_ctx_t
parameter passed to the reducers via the generic ctx field
of the reduce/rereduce function prototype.

Change-Id: I6375033a4c1a661c7eecc5515523bdac817c8823
Reviewed-on: http://review.couchbase.org/26672
Reviewed-by: Fulu Li <fulu@couchbase.com>
Reviewed-by: Aaron Miller <apage43@ninjawhale.com>
Tested-by: Aaron Miller <apage43@ninjawhale.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>