Update mooncake-common/src/default_config.cpp

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: xiaguan

mooncake-common/src/default_config.cpp

@@ -47,6 +47,9 @@ void DefaultConfig::loadFromJSON() {
     Json::Value root;
     std::ifstream file;
     file.open(path_);
+    if (!file.is_open()) {
+        throw std::runtime_error("Failed to open JSON file: " + path_);
+    }
 
     // Read entire file into string
     std::string json_content((std::istreambuf_iterator<char>(file)),