FWIW, not only the region, but the rack controller should surface that it is running out of space:

2017-07-20 05:05:55 sstreams: [info] maas:v2:download/maas:boot:centos:amd64:generic:centos7image26: to_add=['20170720'] to_remove=[]
2017-07-20 05:05:55 sstreams: [info] maas:v2:download/maas:boot:centos:amd64:generic:centos7image28: to_add=['20170720'] to_remove=[]
2017-07-20 05:06:05 twisted.internet.defer: [critical] Unhandled error in Deferred:
2017-07-20 05:06:05 twisted.internet.defer: [critical] 

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 434, in errback
    self._startRunCallbacks(fail)
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 501, in _startRunCallbacks
    self._runCallbacks()
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 588, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1184, in gotResult
    _inlineCallbacks(r, g, deferred)
--- <exception caught here> ---
  File "/usr/lib/python3/dist-packages/twisted/internet/defer.py", line 1126, in _inlineCallbacks
    result = result.throwExceptionIntoGenerator(g)
  File "/usr/lib/python3/dist-packages/twisted/python/failure.py", line 389, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
  File "/usr/lib/python3/dist-packages/provisioningserver/rpc/boot_images.py", line 145, in _import_boot_images
    imported = yield deferToThread(_run_import, sources, **proxies)
  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 232, in wrapper
    result = func(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/provisioningserver/rpc/boot_images.py", line 119, in _run_import
    imported = boot_resources.import_images(sources)
  File "/usr/lib/python3/dist-packages/provisioningserver/import_images/boot_resources.py", line 322, in import_images
    sources, storage, product_mapping)
  File "/usr/lib/python3/dist-packages/provisioningserver/import_images/download_resources.py", line 397, in download_all_boot_resources
    keyring_file=source.get('keyring')),
  File "/usr/lib/python3/dist-packages/provisioningserver/import_images/download_resources.py", line 343, in download_boot_resources
    writer.sync(reader, rpath)
  File "/usr/lib/python3/dist-packages/simplestreams/mirrors/__init__.py", line 91, in sync
    return self.sync_index(reader, path, data, content)
  File "/usr/lib/python3/dist-packages/simplestreams/mirrors/__init__.py", line 254, in sync_index
    self.sync(reader, path=epath)
  File "/usr/lib/python3/dist-packages/simplestreams/mirrors/__init__.py", line 89, in sync
    return self.sync_products(reader, path, data, content)
  File "/usr/lib/python3/dist-packages/simplestreams/mirrors/__init__.py", line 341, in sync_products
    self.insert_item(item, src, target, pgree, ipath_cs)
  File "/usr/lib/python3/dist-packages/provisioningserver/import_images/download_resources.py", line 287, in insert_item
    self.store, filename, tag, checksums, size, contentsource)
  File "/usr/lib/python3/dist-packages/provisioningserver/import_images/download_resources.py", line 63, in insert_file
    store.insert(tag, content_source, checksums, mutable=False, size=size)
  File "/usr/lib/python3/dist-packages/simplestreams/objectstores/__init__.py", line 152, in insert
    wfp.write(buf)
builtins.OSError: [Errno 28] No space left on device