refactor: refactor json loader

1. Fix access level
2. Use error messages in English
3. Throw error if failed

Author: pine

Sources/HolidayJp/HolidayJSONLoader.swift

@@ -1,21 +1,21 @@
 import Foundation
 
-public class HolidayJSONLoader {
-    public static func loadHolidays(fromResource resource: String = "holidays_detailed", bundle: Bundle = .main) -> [String: Holiday] {
+class HolidayJSONLoader {
+    static func loadHolidays(
+        fromResource resource: String = "holidays_detailed",
+        bundle: Bundle = .main
+    ) -> [String: Holiday] {
         guard let url = bundle.url(forResource: resource, withExtension: "json") else {
-            print("[HolidayJSONLoader] JSONファイルが見つかりません: \(resource).json")
-            return [:]
+            fatalError("[HolidayJSONLoader] Unable to find the JSON file: \(resource).json")
         }
         guard let data = try? Data(contentsOf: url) else {
-            print("[HolidayJSONLoader] JSONファイルの読み込みに失敗しました")
-            return [:]
+            fatalError("[HolidayJSONLoader] Failed to load the JSON data: \(url)")
         }
         do {
             let holidays = try JSONDecoder().decode([String: Holiday].self, from: data)
             return holidays
         } catch {
-            print("[HolidayJSONLoader] JSONのデコードに失敗しました: \(error)")
-            return [:]
+            fatalError("[HolidayJSONLoader] Failed to decode the JSON data: \(error)")
         }
     }
 }