Ensure errors about invalid OpenAI API keys are properly surfaced

[jlewi]

If requests fail because of invalid OpenAI API keys (e.g #109) we ensure a clear actionable message is surfaced to the user.
See also #82

[jlewi]

This doesn't appear to be an issue with HashiCorp's retryable errors swallowing the errors.
If the APIKey is invalid OpenAI returns status code status code: 401.
https://platform.openai.com/docs/guides/error-codes/api-errors

It looks like HashiCorp's base retry polies retries on
StatusTooManyRequests - 429
StatusServiceUnavailable - 500 errors with the exception of NotImplemented
https://github.com/hashicorp/go-retryablehttp/blob/86e852df43aa0d94150c4629d74e5116d1ff3348/client.go#L495

One problem is that in the server in Agent we don't log the error if there is one for generate

foyle/app/pkg/agent/agent.go

Line 93 in 3ab5177

return nil, err