LU-1538 tests: improve sanityn test_51 error messages

LU-1538 tests: improve sanityn test_51 error messages

While changing sanityn.sh it caused test_51[abc] to fail in an
obscure manner, and no error messages were being printed or
reporting test failures.

Improve the messages in these tests and fix some code style.

Test-Parameters: trivial testlist=sanityn
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I798f6a7f5ab40714e867b537f4e32e59ea500c1e
Reviewed-on: https://review.whamcloud.com/5256
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Wei Liu <wei3.liu@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Saurabh Tandan <saurabh.tandan@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>