add error handler middleware

Foxchat.Core/Models/Http/ApiError.cs

@@ -0,0 +1,28 @@
+using Newtonsoft.Json;
+using Newtonsoft.Json.Converters;
+
+namespace Foxchat.Core.Models.Http;
+
+public record ApiError
+{
+    public required int Status { get; init; }
+    [JsonConverter(typeof(StringEnumConverter))]
+    public required ErrorCode Code { get; init; }
+    public required string Message { get; init; }
+    [JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
+    public ApiError? OriginalError { get; init; }
+    [JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
+    public string[]? Scopes { get; init; }
+}
+
+public enum ErrorCode
+{
+    InternalServerError,
+    Unauthorized,
+    Forbidden,
+    BadRequest,
+    OutgoingFederationError,
+    AuthenticationError,
+    // TODO: more specific API error codes
+    GenericApiError,
+}