[azopenai] 400 errors from azopenai (for image generation) not returning as HTTP errors

[parkplusplus]

If there's a 400 error from the azopenai SDK it doesn't properly return the error to the user.

Response] ==> REQUEST/RESPONSE (Try=1/213.353709ms, OpTime=213.360126ms) -- RESPONSE RECEIVED
   POST https://api.openai.com/v1/images/generations
   Accept: application/json
   Authorization: REDACTED
   Content-Length: 60
   Content-Type: application/json
   User-Agent: azsdk-go-azopenai/v0.1.0 (go1.20.5; linux)
   --------------------------------------------------------------------------------
   RESPONSE Status: 400 Bad Request
   Access-Control-Allow-Origin: REDACTED

[parkplusplus]

The request was actually well formed - I've just hit my billing limit. So it's a really important error to be able to see.