Proposal: Add warning to user if the API key for selected provider does not exist

What version of Codex is running?

v0.1.2504211509

Which model were you using?

None

What platform is your computer?

Microsoft Windows NT 10.0.26100.0 x64

What steps can reproduce the bug?

Use the --provider flag and choose a provider (do not add <provider>_API_KEY=******* to .env file) and input prompt. Nothing seems to go wrong, but the AI does not return response.

What is the expected behavior?

AI returns a valid response

What do you see instead?

Nothing.

Additional information

I think it would be great that some kind of warning showed up saying, "The api key does not exist for your chosen provider. Please consult https://www.github.com/openai/codex for more info."