Fix potential image_exist bool desync with reality #535

In routes/news_router.py, we set image_exist to true when an image is uploaded (in post_news_image). Bit if we get an error while writing the image to disk, this bool is still set to true which causes a desync with reality. The way things work now this is not a large issue, but it's always good to be able to trust fields like this. I expect a try/except statement around the file writing might be sufficient, capturing OSError.