zdtm: introduce and use pr_err()

Sometimes pr_perror() is used in places that don't have errno set.
In such places, pr_perror() should not be used as it will print
some nonsense.

Introduce and use pr_err() for such cases.

Now, I see sometimes fail() is used in such cases but I think
it is reserved for places where test fails (rather than there is
something that prevents the test from working).

Also, sometimes test_msg() is used but it doesn't print source
file name and line number.

If there will be a need, we can unify pr_err() with say fail(),
it will be easier to do after this patch.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Andrew Vagin <avagin@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>