MB-11538: Give more meaningful error message

Since we use a different directory structure in 3.0, we won't
support to mix older version of backup data with new release.

Change-Id: I7a6e6f180df77e27f11eb487048d75d94718090d
Reviewed-on: http://review.couchbase.org/38846
Reviewed-by: Steve Yen <steve.yen@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>