LU-2826 tests: add useful text to error() calls

Many tests just call "error" without any arguments, but this does not
provide any information to Maloo about why the test fails.  This
in turn causes autovet to match the empty failure message to many
different and irrelevant bugs, making Maloo statistics inaccurate.

Change a number of error() calls to have some valid error text, and
print something more useful if error() is called without arguments.
Not all of the error() calls in sanity.sh and sanityn.sh have been
converted, but it is at least a good start at cleaning this up, and
catches the most frequent failure cases currently.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Liu Ying <emoly.liu@intel.com>
Change-Id: Ie148e3e26e1ecb888ac021a8df7cd995183ebbe5
Reviewed-on: http://review.whamcloud.com/5750
Tested-by: Hudson
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>