[gbdt] enhance error handling for forced splits file loading

Author: KYash03

src/boosting/gbdt.cpp

@@ -83,10 +83,20 @@ void GBDT::Init(const Config* config, const Dataset* train_data, const Objective
   // load forced_splits file
   if (!config->forcedsplits_filename.empty()) {
     std::ifstream forced_splits_file(config->forcedsplits_filename.c_str());
+    if (!forced_splits_file.good()) {
+      Log::Fatal("Could not open forced splits file: %s", config->forcedsplits_filename.c_str());
+    }
     std::stringstream buffer;
     buffer << forced_splits_file.rdbuf();
     std::string err;
     forced_splits_json_ = Json::parse(buffer.str(), &err);
+    if (!err.empty()) {
+      Log::Fatal("Failed to parse forced splits file %s. JSON error: %s", 
+                 config->forcedsplits_filename.c_str(), err.c_str());
+    }
+    if (forced_splits_json_.size() == 0) {
+      Log::Warning("Empty forced splits file: %s", config->forcedsplits_filename.c_str());
+    }
   }
 
   objective_function_ = objective_function;