Improve error message when `OPENAI_API_KEY` env variable is not set.

[sangee2004]

Executing any gptscript without having OPENAI_API_KEY env set results in the following error message:

Sent content:
<content>

Waiting for model response...
2024/02/09 15:35:25 error, status code: 401, message: You didn't provide an API key. You need to provide your API key in an Authorization header using Bearer auth (i.e. Authorization: Bearer YOUR_KEY), or as the password field (with blank username) if you're accessing the API from your browser and are prompted for a username and password. You can obtain an API key from https://platform.openai.com/account/api-keys.

This error message can be improved to instruct user to set OPENAI_API_KEY env variable.
Also this error message can be presented to the user even before making an OpenAPI call.

[saiyam1814]

@ibuildthecloud this is fixed and merged, can be closed

[sangee2004]

Tested with gptscript - version v0.1.0-bbf61d32-dirty

Error message now instruct user to set OPENAI_API_KEY

gptscript https://get.gptscript.ai/echo.gpt --input 'Hello, World!'
2024/02/15 16:57:32 OPENAI_API_KEY is not set. Please set the OPENAI_API_KEY environment variable