zdtm: add more logging messages

Tests fail sometimes, but logs don't contain enough information:
Execute zdtm/live/static/umask00
./umask00 --pidfile=umask00.pid --outfile=umask00.out --mask=0345
make[3]: *** [umask00.pid] Error 1
ERROR: fail to start zdtm/live/static/umask00

Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>