MB-12840: Disable to backup 2.x with 3.x tool

Instead of reverting back to TAP protocal, simple print out
error message that we won't use TAP to backup 2.x cluster
from 3.x tool.

Change-Id: I0d4eee70bd3eba826bd945c7a7c4c67e5f779a5e
Reviewed-on: http://review.couchbase.org/43912
Reviewed-by: Bin Cui <bin.cui@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>