fix: improve error message for unsupported image generation models

relay/channel/gemini/adaptor.go

@@ -59,7 +59,7 @@ func (a *Adaptor) ConvertAudioRequest(c *gin.Context, info *relaycommon.RelayInf
 
 func (a *Adaptor) ConvertImageRequest(c *gin.Context, info *relaycommon.RelayInfo, request dto.ImageRequest) (any, error) {
 	if !strings.HasPrefix(info.UpstreamModelName, "imagen") {
-		return nil, errors.New("not supported model for image generation")
+		return nil, errors.New("not supported model for image generation, only imagen models are supported")
 	}
 
 	// convert size to aspect ratio but allow user to specify aspect ratio