Unmarshal the SubmitBlock error.

The string needs to be unmarshaled rather than returned directly otherwise
it will have an extra set of quotes plus it's safter to properly unmarshal
it.

Pointed out by @jrick.

mining.go

@@ -375,9 +375,14 @@ func (r FutureSubmitBlockResult) Receive() error {
 		return err
 	}
 
-	resStr := string(res)
-	if resStr != "null" {
-		return errors.New(resStr)
+	if string(res) != "null" {
+		var result string
+		err = json.Unmarshal(res, &result)
+		if err != nil {
+			return err
+		}
+
+		return errors.New(result)
 	}
 
 	return nil