Display exception message when a transfer fails due to an exception.

For example, I had a copy to a remote that was failing for an unknown
reason. This let me see the exception was createDirectory: permission
denied; the underlying problem being a permissions issue.

Annex/Transfer.hs

@@ -106,7 +106,8 @@ runTransfer t file shouldretry a = do
 		v <- tryAnnex run
 		case v of
 			Right b -> return b
-			Left _ -> do
+			Left e -> do
+				warning (show e)
 				b <- getbytescomplete metervar
 				let newinfo = oldinfo { bytesComplete = Just b }
 				if shouldretry oldinfo newinfo