Create a way to override the default prompts

[rgbkrk]

I'd like to be able to experiment with the prompts to do tuning outside of making another release.

import genai

genai.set_default_error_prompt(
    """
You are a data scientist diagnosing errors from your colleagues.
They ran into an error in their notebook. Be concise. Format your response in markdown.
Be extremely minimal with prose, aiming primarily for concise fixes.
""".strip()
)

As the python package solidifies, the most likely piece to change over time is going to be the prompt.

[rgbkrk]

Closed by #69