googleai: return err not log.Fatal when stream get error

[Abirdcfly]

the log.Fatal will print the error and exit the program. https://pkg.go.dev/log#Fatal

Fatal is equivalent to Print followed by a call to os.Exit(1).

But the user may have written a series of error handling functions, and I think we should throw the error to the user to deal with, rather than forcing the user's program to stop.

One error that can be reproduced manually is when the request rate limit of the gemini is exceeded.

example: (https://github.com/Abirdcfly/langchaingo-google-stream-exit-early)

// Set the API_KEY env var to your API key taken from ai.google.dev
package main

import (
	"context"
	"fmt"
	"log"
	"os"

	"github.com/tmc/langchaingo/llms"
	"github.com/tmc/langchaingo/llms/googleai"
)

func main() {
	ctx := context.Background()
	apiKey := os.Getenv("API_KEY")
	llm, err := googleai.New(ctx, googleai.WithAPIKey(apiKey))
	if err != nil {
		log.Fatalf("Failed to create client: %v", err)
	}

	prompts := make([]string, 60)
	for i := 1; i <= 60; i++ {
		prompts[i-1] = fmt.Sprintf("Who was the %d person to walk on the moon?", i)
	}
	for _, prompt := range prompts {
		fmt.Printf("\nHuman:\t%s\nstream:\t", prompt)
		answer, err := llms.GenerateFromSinglePrompt(ctx, llm, prompt, llms.WithStreamingFunc(func(ctx context.Context, chunk []byte) error {
			fmt.Print(string(chunk) + ".")
			return nil
		}))
		if err != nil {
			log.Fatalf("Failed to generate: %v, the prompt is:%s", err, prompt)
		}
		fmt.Printf("\nAI:\t%s\n", answer)
	}
}

when before current pull request: (https://github.com/Abirdcfly/langchaingo-google-stream-exit-early/actions/runs/8228267597/job/22497418099#step:4:516)

Human:	Who was the 23 person to walk on the moon?
2024/03/11 05:50:50 googleapi: Error 500:
exit status 1
stream:	

when after current pull request:(https://github.com/Abirdcfly/langchaingo-google-stream-exit-early/actions/runs/8228342012/job/22497623326#step:4:428)

Human:	Who was the 1 person to walk on the moon?
2024/03/11 06:01:40 Failed to generate: error in stream mode: googleapi: Error 500:, the prompt is:Who was the 1 person to walk on the moon?
exit status 1
stream:	

[Abirdcfly]

cc @eliben @tmc

[eliben]

Thanks