[BUG] Active maintenance task list crashes when tasks contain deleted IP devices

[lunkwill42]

Describe the bug
If any IP device attached to an existing and active maintenance task is deleted from NAV, the active maintenance task view will crash with a AttributeError.

To Reproduce
Steps to reproduce the behavior:

1. Create a temporary IP Device called "A"
2. Add A to a maintenance task (e.g. immediate maintenance)
3. Go to the maintenance tool and click the "Active tab"
4. See error

Expected behavior
A list of active maintenance tasks should be displayed, no matter whether they have missing components.

Traceback

Internal Server Error: /maintenance/active/

AttributeError at /maintenance/active/
'DoesNotExist' object has no attribute 'message'

Request Method: GET
Request URL: https://nav.example.org/maintenance/active/
Django Version: 1.11.26
Python Executable: /usr/bin/uwsgi-core
Python Version: 3.5.3
Python Path: ['.', '', '/opt/venvs/nav/lib/python35.zip', '/opt/venvs/nav/lib/python3.5', '/opt/venvs/nav/lib/python3.5/plat-x86_64-linux-gnu', '/opt/venvs/nav/lib/python3.5/lib-dynload', '/usr/lib/python3.5', '/usr/lib/python3.5/plat-x86_64-linux-gnu', '/opt/venvs/nav/lib/python3.5/site-packages', '/usr/local/lib/python3.5/dist-packages', '/usr/lib/python3/dist-packages', '/opt/venvs/nav/lib/python3.5/site-packages', '/etc/nav/python']
Server time: Wed, 18 Dec 2019 09:37:04 +0100
Installed Applications:
('nav.models',
 'nav.web',
 'nav.django',
 'django.contrib.staticfiles',
 'django.contrib.sessions',
 'django.contrib.humanize',
 'django_filters',
 'crispy_forms',
 'crispy_forms_foundation',
 'rest_framework',
 'nav.auditlog',
 'nav.web.macwatch',
 'nav.web.geomap',
 'nav.web.portadmin',
 'django.contrib.postgres')
Installed Middleware:
('django.middleware.common.CommonMiddleware',
 'django.contrib.sessions.middleware.SessionMiddleware',
 'nav.web.auth.AuthenticationMiddleware',
 'nav.web.auth.AuthorizationMiddleware',
 'nav.django.legacy.LegacyCleanupMiddleware',
 'django.contrib.messages.middleware.MessageMiddleware')


Traceback:  

File "/opt/venvs/nav/lib/python3.5/site-packages/nav/web/maintenance/views.py" in active
  130.                     netbox = Netbox.objects.get(pk=int(netbox_id))

File "/opt/venvs/nav/lib/python3.5/site-packages/django/db/models/manager.py" in manager_method
  85.                 return getattr(self.get_queryset(), name)(*args, **kwargs)

File "/opt/venvs/nav/lib/python3.5/site-packages/django/db/models/query.py" in get
  380.                 self.model._meta.object_name
  
    
      During handling of the above exception (Netbox matching query does not exist.), another exception occurred:
    
  

File "/opt/venvs/nav/lib/python3.5/site-packages/django/core/handlers/exception.py" in inner
  41.             response = get_response(request)

File "/opt/venvs/nav/lib/python3.5/site-packages/django/core/handlers/base.py" in _get_response
  187.                 response = self.process_exception_by_middleware(e, request)

File "/opt/venvs/nav/lib/python3.5/site-packages/django/core/handlers/base.py" in _get_response
  185.                 response = wrapped_callback(request, *callback_args, **callback_kwargs)

File "/opt/venvs/nav/lib/python3.5/site-packages/nav/web/maintenance/views.py" in active
  133.                                   netbox_id, get_ex.message)

Exception Type: AttributeError at /maintenance/active/
Exception Value: 'DoesNotExist' object has no attribute 'message'

Environment (please complete the following information):

• NAV version installed: 5.0.2