refactor: add response message to error

Sources/LoggingLoki/LokiError.swift

@@ -1,3 +1,14 @@
-enum LokiError: Error {
-    case invalidResponse
+import struct Foundation.Data
+import protocol Foundation.LocalizedError
+
+enum LokiError: LocalizedError {
+    case invalidResponse(Data?)
+
+    var errorDescription: String? {
+        switch self {
+        case .invalidResponse(let data):
+            guard let data else { return nil }
+            return String(data: data, encoding: .utf8)
+        }
+    }
 }

Sources/LoggingLoki/LokiSession.swift

@@ -56,7 +56,7 @@ extension URLSession: LokiSession {
                 } else if let httpResponse = response as? HTTPURLResponse, (200..<300).contains(httpResponse.statusCode) {
                     completion(.success(httpResponse.statusCode))
                 } else {
-                    completion(.failure(LokiError.invalidResponse))
+                    completion(.failure(LokiError.invalidResponse(data)))
                 }
             }
             task.resume()