Setting discovery parent returns cryptic error

[Huw Wilkins]

To reproduce:
- Visit http://bolla.internal:5240/MAAS/r/dashboard
- Expand a discovery.
- Choose a parent in the select box.
- Click Save.
- See that you get an error which is just a string of random characters (which are different each time you submit).

[Adam Collard (adam-collard)]

2021-06-24 08:57:53 maasserver.websockets.protocol: [critical] Error on request (33) device.create: cy46tb
        Traceback (most recent call last):
          File "/usr/lib/python3.8/threading.py", line 870, in run
            self._target(*self._args, **self._kwargs)
          File "/snap/maas/15048/lib/python3.8/site-packages/provisioningserver/utils/twisted.py", line 835, in worker
            return target()
          File "/snap/maas/15048/usr/lib/python3/dist-packages/twisted/_threads/_threadworker.py", line 46, in work
            task()
          File "/snap/maas/15048/usr/lib/python3/dist-packages/twisted/_threads/_team.py", line 190, in doWork
            task()
        --- <exception caught here> ---
          File "/snap/maas/15048/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 250, in inContext
            result = inContext.theWork()
          File "/snap/maas/15048/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 266, in <lambda>
            inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
          File "/snap/maas/15048/usr/lib/python3/dist-packages/twisted/python/context.py", line 122, in callWithContext
            return self.currentContext().callWithContext(ctx, func, *args, **kw)
          File "/snap/maas/15048/usr/lib/python3/dist-packages/twisted/python/context.py", line 85, in callWithContext
            return func(*args,**kw)
          File "/snap/maas/15048/lib/python3.8/site-packages/provisioningserver/utils/twisted.py", line 870, in callInContext
            return func(*args, **kwargs)
          File "/snap/maas/15048/lib/python3.8/site-packages/provisioningserver/utils/twisted.py", line 202, in wrapper
            result = func(*args, **kwargs)
          File "/snap/maas/15048/lib/python3.8/site-packages/maasserver/utils/orm.py", line 756, in call_within_transaction
            return func_outside_txn(*args, **kwargs)
          File "/snap/maas/15048/lib/python3.8/site-packages/maasserver/utils/orm.py", line 559, in retrier
            return func(*args, **kwargs)
          File "/usr/lib/python3.8/contextlib.py", line 75, in inner
            return func(*args, **kwds)
          File "/snap/maas/15048/lib/python3.8/site-packages/maasserver/websockets/base.py", line 402, in prep_user_execute
            return self._call_method_track_queries(
          File "/snap/maas/15048/lib/python3.8/site-packages/maasserver/websockets/base.py", line 426, in _call_method_track_queries
            result = method(params)
          File "/snap/maas/15048/lib/python3.8/site-packages/maasserver/websockets/handlers/device.py", line 301, in create
            data = super().create(params)
          File "/snap/maas/15048/lib/python3.8/site-packages/maasserver/websockets/base.py", line 507, in create
            return self.full_dehydrate(self.refetch(obj))
          File "/snap/maas/15048/lib/python3.8/site-packages/maasserver/websockets/base.py", line 666, in refetch
            return self.get_object({self._meta.pk: getattr(obj, self._meta.pk)})
          File "/snap/maas/15048/lib/python3.8/site-packages/maasserver/websockets/base.py", line 321, in get_object
            ...

[Adam Collard (adam-collard)]

{u'params': {u'domain': {u'updated': u'Tue, 03 Mar. 2020 15:29:36', u'displayname': u'maas (default)', u'name': u'maas', u'resource_count': 83, u'created': u'Thu, 13 Feb. 2020 14:55:37', u'authoritative': True, u'is_default': True, u'hosts': 8, u'ttl': None, u'id': 0, u'$selected': False}, u'goTo': True, u'parent': u'ytb4wk', u'extra_macs': [], u'saved': False, u'interfaces': [{u'ip_assignment': u'dynamic', u'subnet': 11, u'mac': u'00:16:3e:dd:76:d4', u'ip_address': u'10.1.238.197'}], u'hostname': u'', u'ip_assignment': u'dynamic', u'primary_mac': u'00:16:3e:dd:76:d4', u'deviceIPOptions': [[u'static', u'Static'], [u'dynamic', u'Dynamic'], [u'external', u'External']], u'type': u'device'}, u'type': 0, u'method': u'device.create', u'request_id': 33}