b015bf0676
In 000dec3c6f6e92ec20cb86d1375ec82d2f6062b3, which was only intended to be a refactoring commit, the behavior of this block subtly changed so that unknown types of errors would be swallowed instead of propagated. I noticed this while investigating an error similar to #1539 aka docker/docker#21290. It appears that during GetContent() for a hashstate, the Swift proxy produces an error. Since this error was silently swallowed, an empty []byte is used to restart the hash, then producing the digest of the empty string instead of the layer's digest. This PR will not fix the issue, but it should make the actual error more visible by propagating it into `blobWriter#resumeDigest' and 'blobWriter#validateBlob', respectively. Signed-off-by: Stefan Majewsky <stefan.majewsky@sap.com>