A small fix for extracting the md5 from a chunk file name on S3.
A small fix for extracting the md5 from a chunk file name on S3.

The existing method is preserved. If it fails, then it will fallback
to trying to find the last md5 before the '.sc' extension in the
file name.

trec-kba/streamcorpus-pipeline,trec-kba/streamcorpus-pipeline,trec-kba/streamcorpus-pipeline