Fix error with long messages breaking for everyone

[Alex-Welsh]

The error went like this:
User submits long message, backend returns BadRequestError, INCLUDE_SYSTEM_PROMPT gets flipped, chat history breaks for everyone.

These changes catch the error properly and tell the user to refresh the page.

[sd109]

Having looked more closely at the changes I made to the app.py script while upgrading to Gradio 5, I think the correct fix here is actually to replace the else with an elif len(history) > 0 clause here.

Could you give that a try and see if it handles long message errors better without us having to add extra error handling based on the message text (which is fragile since it'll break if vLLM ever change their error message format)