Better errors

Now you'll receive much better errors if manifest runs without required environment variables. For example:

manifest.py error:

No LLM api keys found, try defining one of the following environment variables
in a .env file or in your environment, then re-running the program:

  - OPENAI_API_KEY

For advanced users, you may manually initialize the LLM client in your code by
calling `manifest.init(client_maker)`, where `client_maker` is a function that
returns an LLM client.

Exiting.