Add user-friendly responses on the GUI

User-friendly errors should be shown to the user on the GUI.

Here's one example: using an OpenAI key with an insufficient quota gives this error on the ExecGPT backend:
`exception: Error code: 429 - {'error': {'message': 'You exceeded your current quota, please check your plan and billing details. For more information on this error, read the docs: https://platform.openai.com/docs/guides/error-codes/api-errors.', 'type': 'insufficient_quota', 'param': None, 'code': 'insufficient_quota'}}`

The **Answer** on the GUI is empty.

One suggestion would be to display user-friendly errors in a pop-up alert (or something similar).