[BUG] Creating a quarantine rule for a vlan that already has a quarantine rule causes 500 error

[stveit]

Describe the bug

If you create a quarantine rule in Arnold (/arnold/addquarantinevlan/) for a vlan that already has a quarantine rule, you get a 500 error (See traceback for details)

To Reproduce

Steps to reproduce the behavior: (replace X with any number you want to use, e.g. 100)

1. Go to/arnold/addquarantinevlan/
2. Enter vlan X into the vlan input box on the right
3. Click Add vlan
4. Enter vlan X into the vlan input box on the right
5. Click Add vlan
6. See error

Expected behavior

Since the same Vlan cannot be quarantined twice, I would expect a non-500 error saying that plainly beneath the form or something.

Screenshots

Tracebacks

Traceback (most recent call last):
  File "/opt/venvs/nav/lib/python3.9/site-packages/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)

The above exception (duplicate key value violates unique constraint "quarantine_vlan_unique"
DETAIL:  Key (vlan)=(1) already exists.
) was the direct cause of the following exception:
  File "/opt/venvs/nav/lib/python3.9/site-packages/django/core/handlers/exception.py", line 47, in inner
    response = get_response(request)
  File "/opt/venvs/nav/lib/python3.9/site-packages/django/core/handlers/base.py", line 181, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/source/python/nav/web/arnold/views.py", line 462, in render_quarantine_vlans
    process_quarantinevlan_form(form)
  File "/source/python/nav/web/arnold/views.py", line 502, in process_quarantinevlan_form
    qvlan.save()
  File "/opt/venvs/nav/lib/python3.9/site-packages/django/db/models/base.py", line 739, in save
    self.save_base(using=using, force_insert=force_insert,
  File "/opt/venvs/nav/lib/python3.9/site-packages/django/db/models/base.py", line 776, in save_base
    updated = self._save_table(
  File "/opt/venvs/nav/lib/python3.9/site-packages/django/db/models/base.py", line 881, in _save_table
    results = self._do_insert(cls._base_manager, using, fields, returning_fields, raw)
  File "/opt/venvs/nav/lib/python3.9/site-packages/django/db/models/base.py", line 919, in _do_insert
    return manager._insert(
  File "/opt/venvs/nav/lib/python3.9/site-packages/django/db/models/manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/opt/venvs/nav/lib/python3.9/site-packages/django/db/models/query.py", line 1270, in _insert
    return query.get_compiler(using=using).execute_sql(returning_fields)
  File "/opt/venvs/nav/lib/python3.9/site-packages/django/db/models/sql/compiler.py", line 1416, in execute_sql
    cursor.execute(sql, params)
  File "/opt/venvs/nav/lib/python3.9/site-packages/django/db/backends/utils.py", line 98, in execute
    return super().execute(sql, params)
  File "/opt/venvs/nav/lib/python3.9/site-packages/django/db/backends/utils.py", line 66, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File "/opt/venvs/nav/lib/python3.9/site-packages/django/db/backends/utils.py", line 75, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "/opt/venvs/nav/lib/python3.9/site-packages/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
  File "/opt/venvs/nav/lib/python3.9/site-packages/django/db/utils.py", line 90, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "/opt/venvs/nav/lib/python3.9/site-packages/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)

Exception Type: IntegrityError at /arnold/addquarantinevlan/
Exception Value: duplicate key value violates unique constraint "quarantine_vlan_unique"
DETAIL:  Key (vlan)=(1) already exists.