describe: error if blob not found

If describe_blob() does not find the blob in question, it returns an
empty strbuf, and we print an empty line. This differs from
describe_commit(), which always either returns an answer or calls die()
itself. As the blob function was bolted onto the command afterwards, I
think its behavior is not intentional, and it is just a bug that it does
not report an error.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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