LU-2174 test: improve error message

LU-2174 test: improve error message

In sanity-quota.sh, if the testing user/group isn't existing, print
error message to inform user to create them.

Check free space for test_0.

Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: Ie08250d665b305b140315f76391fd5161a6fbdd5
Reviewed-on: http://review.whamcloud.com/4268
Tested-by: Hudson
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>