feat(proxy): throw user-readable error on 413 when sending proxied attachments

Myriad/Rest/BaseRestClient.cs

@@ -262,6 +262,7 @@ public class BaseRestClient: IAsyncDisposable
             HttpStatusCode.Unauthorized => new UnauthorizedException(response, body, apiError),
             HttpStatusCode.NotFound => new NotFoundException(response, body, apiError),
             HttpStatusCode.Conflict => new ConflictException(response, body, apiError),
+            HttpStatusCode.RequestEntityTooLarge => new RequestEntityTooLargeException(response, body, apiError),
             HttpStatusCode.TooManyRequests => new TooManyRequestsException(response, body, apiError),
             _ => new UnknownDiscordRequestException(response, body, apiError)
         };

Myriad/Rest/Exceptions/DiscordRequestException.cs

@@ -61,6 +61,14 @@ public class BadRequestException: DiscordRequestException
     { }
 }
 
+public class RequestEntityTooLargeException: DiscordRequestException
+{
+    public RequestEntityTooLargeException(HttpResponseMessage response, string responseBody, DiscordApiError? apiError) : base(
+        response, responseBody, apiError)
+    { }
+}
+
+
 public class TooManyRequestsException: DiscordRequestException
 {
     public TooManyRequestsException(HttpResponseMessage response, string responseBody, DiscordApiError? apiError) :

PluralKit.Bot/Services/WebhookExecutorService.cs

@@ -179,6 +179,25 @@ public class WebhookExecutorService
                         throw e;
                 }
             }
+            catch (RequestEntityTooLargeException e)
+            {
+                try
+                {
+                    await _rest.CreateMessage(req.ChannelId, new MessageRequest {
+                        Content = $"{Emojis.Error} One or more of the files attached to this message were not able to be proxied because they were too large.",
+                        AllowedMentions = new AllowedMentions { Parse = {} },
+                    });
+
+                    throw new ProxyService.ProxyChecksFailedException("_internal_discord_rejected_message");
+                }
+                catch (Exception ex)
+                {
+                    if (ex.GetType() == typeof(ProxyService.ProxyChecksFailedException))
+                        throw ex;
+                    else
+                        throw e;
+                }
+            }
             catch (JsonReaderException)
             {
                 // This happens sometimes when we hit a CloudFlare error (or similar) on Discord's end