vdsm-tool: handle an exception and return an error code

As an independent cli, it is nicer for vdsm-tool to print an error and
return an appropriate error code on any exception. Letting exception go
unhandled make abrt report it as an application crash.

Bug-Url: https://bugzilla.redhat.com/1054759
Change-Id: I9c7b677ef8de713d2912f8926991b1dcd8134ccf
Signed-off-by: Timothy Asir <tjeyasin@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23551
Reviewed-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-by: Alon Bar-Lev <alonbl@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Tested-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>