Improve "Requested resource does not exist" exception message to give slightly more context

DiscordChatExporter.Core/Discord/DiscordClient.cs

@@ -51,7 +51,7 @@ namespace DiscordChatExporter.Core.Discord
                 {
                     HttpStatusCode.Unauthorized => DiscordChatExporterException.Unauthorized(),
                     HttpStatusCode.Forbidden => DiscordChatExporterException.Forbidden(),
-                    HttpStatusCode.NotFound => DiscordChatExporterException.NotFound(),
+                    HttpStatusCode.NotFound => DiscordChatExporterException.NotFound(url),
                     _ => DiscordChatExporterException.FailedHttpRequest(response)
                 };
             }

DiscordChatExporter.Core/Exceptions/DiscordChatExporterException.cs

@@ -36,8 +36,8 @@ Failed to perform an HTTP request.
         internal static DiscordChatExporterException Forbidden() =>
             new("Access is forbidden.");
 
-        internal static DiscordChatExporterException NotFound() =>
+        internal static DiscordChatExporterException NotFound(string resourceId) =>
-            new("Requested resource does not exist.");
+            new($"Requested resource ({resourceId}) does not exist.");
 
         internal static DiscordChatExporterException ChannelIsEmpty() =>
             new("No messages found for the specified period.");