Bad VLAN Validation on UI

Preventing assigning the second interface to the same fabric renders building bonds impossible from UI.

MAAS Version 2.0.0 (beta2+bzr4920)

2016-04-18 07:47:34 [-] Error on request (72) machine.update_interface: {"vlan": ["A physical interface can only belong to an untagged VLAN."]}
 Traceback (most recent call last):
   File "/usr/lib/python3.5/threading.py", line 862, in run
     self._target(*self._args, **self._kwargs)
   File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 841, in worker
     return target()
   File "/usr/lib/python3/dist-packages/twisted/_threads/_threadworker.py", line 46, in work
     task()
   File "/usr/lib/python3/dist-packages/twisted/_threads/_team.py", line 190, in doWork
     task()
 --- <exception caught here> ---
   File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 246, in inContext
     result = inContext.theWork()
   File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 262, in <lambda>
     inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
   File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 118, in callWithContext
     return self.currentContext().callWithContext(ctx, func, *args, **kw)
   File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 81, in callWithContext
     return func(*args,**kw)
   File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 874, in callInContext
     return func(*args, **kwargs)
   File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 516, in call_within_transaction
     return func_outside_txn(*args, **kwargs)
   File "/usr/lib/python3/dist-packages/maasserver/utils/orm.py", line 343, in retrier
     return func(*args, **kwargs)
   File "/usr/lib/python3.5/contextlib.py", line 30, in inner
     return func(*args, **kwds)
   File "/usr/lib/python3/dist-packages/maasserver/websockets/handlers/machine.py", line 751, in update_interface
     raise ValidationError(form.errors)
 django.core.exceptions.ValidationError: {'vlan': ['A physical interface can only belong to an untagged VLAN.']}