Adding a device with a MAC address that's belongs to another node, fails without providing any feedback

[Andres Rodriguez]

When trying to add a device that had a MAC owned by another device, it failed without providing any user feedback in the UI. However, the following was provided in the log:

2016-01-13 23:23:37 [-] Error on request (35) device.create: {"mac_addresses": ["MAC address aa:bb:cc:dd:ee:f1 already in use on testone."]}
 Traceback (most recent call last):
   File "/usr/lib/python2.7/threading.py", line 783, in __bootstrap
     self.__bootstrap_inner()
   File "/usr/lib/python2.7/threading.py", line 810, in __bootstrap_inner
     self.run()
   File "/usr/lib/python2.7/threading.py", line 763, in run
     self.__target(*self.__args, **self.__kwargs)
   File "/usr/lib/python2.7/dist-packages/provisioningserver/utils/twisted.py", line 791, in _worker
     return super(ThreadPool, self)._worker()
 --- <exception caught here> ---
   File "/usr/lib/python2.7/dist-packages/twisted/python/threadpool.py", line 191, in _worker
     result = context.call(ctx, function, *args, **kwargs)
   File "/usr/lib/python2.7/dist-packages/twisted/python/context.py", line 118, in callWithContext
     return self.currentContext().callWithContext(ctx, func, *args, **kw)
   File "/usr/lib/python2.7/dist-packages/twisted/python/context.py", line 81, in callWithContext
     return func(*args,**kw)
   File "/usr/lib/python2.7/dist-packages/maasserver/utils/orm.py", line 501, in call_within_transaction
     return func_outside_txn(*args, **kwargs)
   File "/usr/lib/python2.7/dist-packages/maasserver/utils/orm.py", line 328, in retrier
     return func(*args, **kwargs)
   File "/usr/lib/python2.7/dist-packages/django/db/transaction.py", line 339, in inner
     return func(*args, **kwargs)
   File "/usr/lib/python2.7/dist-packages/maasserver/websockets/handlers/device.py", line 306, in create
     data = super(DeviceHandler, self).create(params)
   File "/usr/lib/python2.7/dist-packages/maasserver/websockets/base.py", line 364, in create
     raise HandlerValidationError(e.error_dict)
 maasserver.websockets.base.HandlerValidationError: {u'mac_addresses': [u'MAC address aa:bb:cc:dd:ee:f1 already in use on testone.']}

[Lee Trager (ltrager)]

I get an error message in the UI on trunk.