How to get error details correctly when using streaming mode

### Description of the feature request:

Hi:
Thank you for your hard work. I have some questions about the stream mode. This is my code:

```
func testStream() {
ctx := context.Background()
client, err := genai.NewClient(ctx, option.WithAPIKey("You-Key"))
if err != nil {
log.Fatal(err)
}
defer client.Close()

model := client.GenerativeModel("gemini-1.5-flash")
// non-stream
resp, err := model.GenerateContent(ctx, genai.Text("Write a story about a magic backpack."))
if err != nil {
log.Fatal(err)
}

printResponse(resp)

// using stream
// iter := model.GenerateContentStream(ctx,
// genai.Text("Write a story about a magic backpack."),
// )

// for {
// res, err := iter.Next()
// if err == iterator.Done {
// break
// }
// if err != nil {
// log.Fatal(err)
// }
// printResponse(res)
// }
}
func printResponse(resp *genai.GenerateContentResponse) {
for _, cand := range resp.Candidates {
if cand.Content != nil {
for _, part := range cand.Content.Parts {
fmt.Println(part)
}
}
}
fmt.Println("---")
}
```
I want to capture more abnormal scenarios in the code, such as when there is a problem with the key (http code 429) or the parameter (http code 400), I can get the corresponding error information.

When I use non-stream, I can get the error information correctly:
```
googleapi: Error 403: Permission denied: Consumer 'api_key:AIzaSyxxx' has been suspended.
error details: name = ErrorInfo reason = CONSUMER_SUSPENDED domain = googleapis.com metadata = map[consumer:projects/897183970500 containerInfo:api_key:AIzaSyxxxservice:generativelanguage.googleapis.com]
error details: name = LocalizedMessage locale = en-US msg = Permission denied: Consumer 'api_key:AIzaSyxxx' has been suspended.

```

When I use stream, I can only get the http code, but not the corresponding error information:

`googleapi: Error 403:`

### What problem are you trying to solve with this feature?

I need to get the corresponding error information in various scenarios so that I can handle the abnormal scenarios correctly.

I hope I can get this information normally regardless of whether I am streaming or not.

### Any other information you'd like to share?

I tried to cast the error type, but the `errorMessage` is still empty:

```
apiError, ok := err.(*apierror.APIError)
if ok {
statusCode := apiError.HTTPCode()
errorMessage := apiError.GRPCStatus().Message()
}
gerr, ok := err.(*googleapi.Error)
if ok {
statusCode := gerr.Code
errorMessage := gerr.Message
}
```