MB-12693: Check if root backup dirctory is valid

Change-Id: I4ae6ad7d4537ed73dadb5c2e850239d0e9e5b3d3
Reviewed-on: http://review.couchbase.org/43561
Reviewed-by: Bin Cui <bin.cui@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>