Handle case where error response body may not be JSON

[atesgoral]

Fixes #357

Gracefully handle the case where an HTTP error response may not have valid JSON in its body e.g.:

• OpenAI is having a bad day an returns a 500 Internal Server Error
• Some other network element like a proxy is returning some 4xx or 5xx.

All Submissions:

• [x] Have you followed the guidelines in our Contributing document?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [x] Have you added an explanation of what your changes do and why you'd like us to include them?

[alexrudall]

Thanks for this!