Error when lifting detentions crashes Arnold

[jmbredal]

Should probably show a nice error-message to user instead.

Internal Server Error: /arnold/doenable/
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/django/core/handlers/base.py", line 111, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/lib/python2.7/dist-packages/nav/web/arnold/views.py", line 334, in lift_detentions
    open_port(identity, account.login, 'Enabled from web')
  File "/usr/lib/python2.7/dist-packages/nav/arnold.py", line 392, in open_port
    change_port_vlan(identity, identity.fromvlan)
  File "/usr/lib/python2.7/dist-packages/nav/arnold.py", line 466, in change_port_vlan
    raise ChangePortVlanError(error)
ChangePortVlanError: An error occured when changing portvlan: SNMP Error: wrongValue (The set value is illegal or unsupported in some way)