hsm: Log unhandled exceptions in storage refresh thread

hsm's storage refresh background thread does not handle exceptions,
making it harder then it should to debug stupid errors in underlying
calls or debug real errors in the field.

This patch log a traceback for unhandled exceptions so it will never die
silently again.

Change-Id: I2e69464ba27b7d522c5ceb3a3099949efe7fbc88
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21486
Reviewed-by: Dan Kenigsberg <danken@redhat.com>