No UI error if boot-source is invalid

[Jason Hobbs]

I changed the boot-images sync-url setting in the UI to a URL that didn't contain boot-sources. There was no error in the UI, but there were stacktraces in maas-django.log.

Stacktrace:
ERROR 2014-11-14 23:09:53,510 twisted Importing boot resources failed.
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)
--- <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/provisioningserver/utils/twisted.py", line 143, in wrapper
    return func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/maasserver/bootresources.py", line 930, in _import_resources
    image_descriptions = download_all_image_descriptions(sources)
  File "/usr/lib/python2.7/dist-packages/provisioningserver/import_images/download_descriptions.py", line 204, in download_all_image_descriptions
    source['url'], keyring=source.get('keyring', None))
  File "/usr/lib/python2.7/dist-packages/provisioningserver/import_images/download_descriptions.py", line 195, in download_image_descriptions
    dumper.sync(reader, rpath)
  File "/usr/lib/python2.7/dist-packages/simplestreams/mirrors/__init__.py", line 82, in sync
    content, payload = reader.read_json(path)
  File "/usr/lib/python2.7/dist-packages/simplestreams/mirrors/__init__.py", line 39, in read_json
    raw = self.source(path).read().decode('utf-8')
  File "/usr/lib/python2.7/dist-packages/simplestreams/contentsource.py", line 143, in read
    self.open()
  File "/usr/lib/python2.7/dist-packages/simplestreams/contentsource.py", line 139, in open
    self.fd = self._open()
  File "/usr/lib/python2.7/dist-packages/simplestreams/contentsource.py", line 135, in _open
    raise myerr
exceptions.IOError: Unable to open http://maas.ubuntu.com/images/ephemeral-v2/releases/daily/streams/v1/index.sjson. mirrors=[]

1.7.0~rc3+bzr3299-0