Detect error strings on SumitBlock.

The submitblock RPC returns a string containing an error if it failed for
any reason.  This comment detects a non-null return from submitblock and
converts that string to an error which is returned.

mining.go

@@ -369,11 +369,16 @@ type FutureSubmitBlockResult chan *response
 // Receive waits for the response promised by the future and returns an error if
 // any occurred when submitting the block.
 func (r FutureSubmitBlockResult) Receive() error {
-	_, err := receiveFuture(r)
+	res, err := receiveFuture(r)
 	if err != nil {
 		return err
 	}
 
+	resStr := string(res)
+	if resStr != "null" {
+		return errors.New(resStr)
+	}
+
 	return nil
 
 }