cluster/dht: Modified test case to note rename failures as errors

The bug referenced in this change, had an race condition that is now
fixed by the following commits that are posted for review.

    http://review.gluster.org/#/c/8563/
    http://review.gluster.org/#/c/8570/

These changes would now make the winning client not fail a rename,
in case it failed to rename the linkto file. Hence when one client
wins the link race, and the other still deletes the linkto file,
the rename failure by the winning client is not a critical failure,
hence it resolves the issue posted in the bug.

As a result modifying the test case to treat the rename failures
as errors, to catch any future issues.

Change-Id: Ibe9caac7ee87dcbc4f581cfbd36173b734859ccb
BUG: 1123950
Signed-off-by: Shyam <srangana@redhat.com>
Reviewed-on: http://review.gluster.org/8579
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>