LLM::Bot#chat should raise PromptError

### Context

This is a scenario where our API is lacking. 
A `PromptError` should be raised as soon as we are given a prompt object we don't understand:

```ruby
#!/usr/bin/env ruby

llm = LLM.openai key: ENV["OPENAI_SECRET"]
bot = LLM::Bot.new(llm)
bot.chat(Object.new) # PromptError should be raised here
bot.messages.to_a    # But instead it is raised here
```