Fix multi-turn prompting error handling and extra newline

[apaz-cli]

Using .format() may overwrite user input. Rewrote in the style of the llama prompt format, and removed the extra newline from the system prompt.

[rasbt]

Thanks for the contribution!

Based on checking https://llama.meta.com/docs/model-cards-and-prompt-formats/meta-llama-3/, there shouldn't be a newline indeed. Any thoughts @Andrei-Aksionov ?

[Andrei-Aksionov]

No, I don't have any clue why an extra newline was added in the first place.
Additionally, I checked that \n\n separately (like in Meta repo) and in concatenation with other tokens like <|end_header_id|>\n\n is encoded as the same token. Sometimes encode(prompt_1) + encode(prompt_2) != encode(prompt_1 + prompt_2) #1567.

Thanks for the PR @apaz-cli 👍