vdsm-tool: handle error during module loading

While vdsm-tool initiates it loads all python modules under tool folder.
If one of them throws exception vdsm-tool fails to run. This patch
handles such exception and reports it to syslog.

Change-Id: I023ad390723617d10664f9bf5b8d1460131efef2
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23456
Reviewed-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>