Raise an error from ovs_lib list operations

Raise an error from ovs_lib list operations

Previously list operations in ovs_lib returns an empty list
if RuntimeError occurs and a caller cannot distinguish an error
from normal results. This commit changes ovs_lib list operations
(get_vif_port_set, get_vif_ports, get_bridges) to raise an
exception when RuntimeError occurs.

Note: callers of these commands are ovs/nec/ryu-agent and ovs_cleanup.
- plugin agents: these commands are inside in try/except clause
  in daemon loop and there is no need to change.
- ovs_cleanup: there is no error catch logic in main() at now
  and it calls commands other than ovs_lib, so it can be cleanup
  later if required.

It also fixes the code to use excutils.save_and_reraise_exception
when reraising an exception.

Change-Id: I2aa3b51b8661c75846cb588c08c8f8ee00c37004
Closes-Bug: #1277029