feat(apiv2): basic error handling

PluralKit.API/Errors.cs

@@ -0,0 +1,51 @@
+using System;
+
+using Newtonsoft.Json.Linq;
+
+namespace PluralKit.API
+{
+    public class PKError: Exception
+    {
+        public int ResponseCode { get; init; }
+        public int JsonCode { get; init; }
+        public PKError(int code, int json_code, string message) : base(message)
+        {
+            ResponseCode = code;
+            JsonCode = json_code;
+        }
+
+        public JObject ToJson()
+        {
+            var j = new JObject();
+            j.Add("message", this.Message);
+            j.Add("code", this.JsonCode);
+            return j;
+        }
+    }
+
+    public class ModelParseError: PKError
+    {
+        public ModelParseError() : base(400, 0, "Error parsing JSON model")
+        {
+            // todo
+        }
+
+        public new JObject ToJson()
+        {
+            var j = base.ToJson();
+
+            return j;
+        }
+    }
+
+    public static class APIErrors
+    {
+        public static PKError GenericBadRequest = new(400, 0, "400: Bad Request");
+        public static PKError SystemNotFound = new(404, 20001, "System not found.");
+        public static PKError MemberNotFound = new(404, 20002, "Member not found.");
+        public static PKError GroupNotFound = new(404, 20003, "Group not found.");
+        public static PKError UnauthorizedMemberList = new(403, 30001, "Unauthorized to view member list");
+        public static PKError UnauthorizedGroupList = new(403, 30002, "Unauthorized to view group list");
+        public static PKError Unimplemented = new(501, 50001, "Unimplemented");
+    }
+}