vdsm-tool: Change exception reporting

General environment issues will raise EnvironmentError, Execution failures
will raise RuntimeError. For specific vdsm-tool usage this patch introduce
vdsm.tool.UsageError as the root class for exceptions to present to the user.
Usage errors are only printed to the screen preceded by "Error:" and do
not generate a stack trace.

Change-Id: I467d1e5ef7503312cde6e013aacb68dc2d46dd37
Signed-off-by: Dima Kuznetsov <dkuznets@redhat.com>
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26565
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>