ethclient, ethereum: add NotFound, split transactions out of ChainReader

ethclient now returns ethereum.NotFound if the server returns null and
no error while accessing blockchain data.

The light client cannot provide arbitrary transactions. The change to
split transaction access into its own interface emphasizes that
transactions should not be relied on and recommends use of logs.

ethclient/ethclient_test.go

@@ -21,9 +21,9 @@ import "github.com/ethereum/go-ethereum"
 // Verify that Client implements the ethereum interfaces.
 var (
 	_ = ethereum.ChainReader(&Client{})
+	_ = ethereum.TransactionReader(&Client{})
 	_ = ethereum.ChainStateReader(&Client{})
 	_ = ethereum.ChainSyncReader(&Client{})
-	_ = ethereum.ChainHeadEventer(&Client{})
 	_ = ethereum.ContractCaller(&Client{})
 	_ = ethereum.GasEstimator(&Client{})
 	_ = ethereum.GasPricer(&Client{})