Add checks for message content intent on bot accounts

Related to #918

DiscordChatExporter.Core/Exceptions/DiscordChatExporterException.cs

@@ -41,4 +41,7 @@ Failed to perform an HTTP request.
 
     internal static DiscordChatExporterException ChannelIsEmpty() =>
         new("No messages found for the specified period.");
+
+    internal static DiscordChatExporterException MessageContentIntentMissing() =>
+        new("Failed to retrieve message content because the bot doesn't have the Message Content Intent enabled.");
 }