refactor: Enhance error handling in Gemini request conversion

- Updated `CovertGemini2OpenAI` function to return an error alongside the GeminiChatRequest, improving error reporting for image processing.
- Modified `ConvertRequest` methods in both `adaptor.go` files to handle potential errors from the Gemini conversion, ensuring robust request handling.
- Improved clarity and maintainability of the code by explicitly managing error cases during request conversion.

relay/channel/gemini/adaptor.go

@@ -57,7 +57,11 @@ func (a *Adaptor) ConvertRequest(c *gin.Context, info *relaycommon.RelayInfo, re
 	if request == nil {
 		return nil, errors.New("request is nil")
 	}
-	return CovertGemini2OpenAI(*request), nil
+	ai, err := CovertGemini2OpenAI(*request)
+	if err != nil {
+		return nil, err
+	}
+	return ai, nil
 }
 
 func (a *Adaptor) ConvertRerankRequest(c *gin.Context, relayMode int, request dto.RerankRequest) (any, error) {

relay/channel/gemini/relay-gemini.go

@@ -17,7 +17,7 @@ import (
 )
 
 // Setting safety to the lowest possible values since Gemini is already powerless enough
-func CovertGemini2OpenAI(textRequest dto.GeneralOpenAIRequest) *GeminiChatRequest {
+func CovertGemini2OpenAI(textRequest dto.GeneralOpenAIRequest) (*GeminiChatRequest, error) {
 	geminiRequest := GeminiChatRequest{
 		Contents: make([]GeminiChatContent, 0, len(textRequest.Messages)),
 		SafetySettings: []GeminiChatSafetySettings{
@@ -110,7 +110,7 @@ func CovertGemini2OpenAI(textRequest dto.GeneralOpenAIRequest) *GeminiChatReques
 				imageNum += 1
 
 				if constant.GeminiVisionMaxImageNum != -1 && imageNum > constant.GeminiVisionMaxImageNum {
-					continue
+					return nil, fmt.Errorf("too many images in the message, max allowed is %d", constant.GeminiVisionMaxImageNum)
 				}
 				// 判断是否是url
 				if strings.HasPrefix(part.ImageUrl.(dto.MessageImageUrl).Url, "http") {
@@ -125,7 +125,7 @@ func CovertGemini2OpenAI(textRequest dto.GeneralOpenAIRequest) *GeminiChatReques
 				} else {
 					_, format, base64String, err := service.DecodeBase64ImageData(part.ImageUrl.(dto.MessageImageUrl).Url)
 					if err != nil {
-						continue
+						return nil, fmt.Errorf("decode base64 image data failed: %s", err.Error())
 					}
 					parts = append(parts, GeminiPart{
 						InlineData: &GeminiInlineData{
@@ -162,7 +162,7 @@ func CovertGemini2OpenAI(textRequest dto.GeneralOpenAIRequest) *GeminiChatReques
 		//	shouldAddDummyModelMessage = false
 		//}
 	}
-	return &geminiRequest
+	return &geminiRequest, nil
 }
 
 func (g *GeminiChatResponse) GetResponseText() string {

relay/channel/vertex/adaptor.go

@@ -135,7 +135,10 @@ func (a *Adaptor) ConvertRequest(c *gin.Context, info *relaycommon.RelayInfo, re
 		c.Set("request_model", request.Model)
 		return vertexClaudeReq, nil
 	} else if a.RequestMode == RequestModeGemini {
-		geminiRequest := gemini.CovertGemini2OpenAI(*request)
+		geminiRequest, err := gemini.CovertGemini2OpenAI(*request)
+		if err != nil {
+			return nil, err
+		}
 		c.Set("request_model", request.Model)
 		return geminiRequest, nil
 	} else if a.RequestMode == RequestModeLlama {