HandlerError suppressed in UI

When creating a RAID device the following error appeared in the log, but
the UI showed no error. The block devices were removed from the "Used
disks and partitions" table but no RAID device appeared in their place.
I had to refresh the page to return things to normal. I would expect the
error to be displayed in the page and the selected block devices to
remain unchanged. I wonder if this is related to bug 1489149?

2016-03-14 13:59:49 [-] Error on request (187) machine.create_raid: <ul
class="errorlist"><li>block_devices<ul class="errorlist"><li>Select a
valid choice. 1 is not one of the available
choices.</li></ul></li></ul>
Traceback (most recent call last):
  File "/usr/lib/python3.5/threading.py", line 862, in run
    self._target(*self._args, **self._kwargs)
  File ".../src/provisioningserver/utils/twisted.py", line 794, 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 ".../src/maasserver/utils/orm.py", line 448, in
  call_within_transaction
    return func_outside_txn(*args, **kwargs)
  File ".../src/maasserver/utils/orm.py", line 275, in retrier
    return func(*args, **kwargs)
  File "/usr/lib/python3.5/contextlib.py", line 30, in inner
    return func(*args, **kwds)
  File ".../src/maasserver/websockets/handlers/machine.py", line 556, in
  create_raid
    raise HandlerError(form.errors)
maasserver.websockets.base.HandlerError: <ul
class="errorlist"><li>block_devices<ul class="errorlist"><li>Select a
valid choice. 1 is not one of the available choices.</li></ul></li></ul>