feat: More verbose forwarding errors (#1523)

Fixes #1516.

tests/common/util/MockLoadBalancer.hpp

@@ -19,6 +19,7 @@
 
 #pragma once
 
+#include "rpc/Errors.hpp"
 #include "util/FakeFetchResponse.hpp"
 
 #include <boost/asio/spawn.hpp>
@@ -28,6 +29,7 @@
 #include <gmock/gmock.h>
 
 #include <cstdint>
+#include <expected>
 #include <optional>
 #include <string>
 
@@ -37,8 +39,10 @@ struct MockLoadBalancer {
     MOCK_METHOD(void, loadInitialLedger, (std::uint32_t, bool), ());
     MOCK_METHOD(std::optional<FakeFetchResponse>, fetchLedger, (uint32_t, bool, bool), ());
     MOCK_METHOD(boost::json::value, toJson, (), (const));
+
+    using ForwardToRippledReturnType = std::expected<boost::json::object, rpc::ClioError>;
     MOCK_METHOD(
-        std::optional<boost::json::object>,
+        ForwardToRippledReturnType,
         forwardToRippled,
         (boost::json::object const&, std::optional<std::string> const&, bool, boost::asio::yield_context),
         (const)