lfs: improve an exception message for blob corruption detected on transfer The message about the server crash originated in 0ee0a3f6a990 (after support for serving blobs was added), but was copied from the Facebook repo that forked prior to server side support. Therefore, this message only displayed in their client, so it was safe to assume the server crashed. But that was never the case for vanilla Mercurial, as I saw this in a server log. Also, display the blob reference so that it's easier to figure out where the problem was when a bunch of blobs are transferred at once.