Fails to notice download error

[Jeroen T. Vermeulen]

We were trying to sync a simplestreams stream into a local ObjectStoreMirrorWriter from behind a proxy that returns a 403 (Forbidden).

Looks like the failure wasn't noticed until our "policy" function tried to verify the signature on the index:

    return util.read_signed(content, keyring=args.keyring)

File "/usr/lib/python2.7/dist-packages/simplestreams/util.py", line 283, in read_signed
    raise SignatureMissingException("No signature found!")
simplestreams.util.SignatureMissingException: No signature found!

We would have expected an exception at the point where the download fails, where it's easier to debug. Instead, ObjectStoreMirrorWriter.sync() continued and handed the error page into the policy function.