openvswitch mech. driver does not report error in check_segment_for_agent

When administrator misspells mappings for external flat networks, nova fails with obscure trace during instance creation:

 Traceback (most recent call last):
   File "/usr/lib/python2.7/dist-packages/nova/compute/manager.py", line 2231, in _build_resources
     yield resources
   File "/usr/lib/python2.7/dist-packages/nova/compute/manager.py", line 2101, in _build_and_run_instance
     block_device_info=block_device_info)
   File "/usr/lib/python2.7/dist-packages/nova/virt/libvirt/driver.py", line 2619, in spawn
     write_to_disk=True)
   File "/usr/lib/python2.7/dist-packages/nova/virt/libvirt/driver.py", line 4150, in _get_guest_xml
     context)
   File "/usr/lib/python2.7/dist-packages/nova/virt/libvirt/driver.py", line 3936, in _get_guest_config
     flavor, CONF.libvirt.virt_type)
   File "/usr/lib/python2.7/dist-packages/nova/virt/libvirt/vif.py", line 352, in get_config
     _("Unexpected vif_type=%s") % vif_type)
 NovaException: Unexpected vif_type=binding_failed

The real problem lies in neutron/plugins/ml2/drivers/mech_openvswitch.py:

        network_type = segment[api.NETWORK_TYPE]
        if network_type == 'local':
            return True
        elif network_type in tunnel_types:
            return True
        elif network_type in ['flat', 'vlan']:
            return segment[api.PHYSICAL_NETWORK] in mappings
        else:
            return False

If network_type is 'flat' and segment[api.PHYSICAL_NETWORK] is not in mappings it returns False, this causes all other problems.

Proposal: add some kind of WARNING in this place to let the administrator know that no matching mappings found.