More informative error messages for failed requests

[AlbertRapp]

Give helpful comments as to why a chat flow didn't work.

Authorization denied: Something wrong with the API Key

404 Not Found: Model not available (At least for openAI this can simply be due to the fact that GPT-4 is only available once billing is added to the API account (not the chatGPT account)

400 Bad Request: Missing or incorrect parameters