Better error when uploaded image is too large

[cellio]

https://meta.codidact.com/posts/284143

If you try to upload an image that is too large, you get a 413 from the web server instead of an in-Codidact message like for some other errors. Ultimately we probably want to resize too-large images and proceed, but we probably won't do that for a while. In the meantime, can we catch this error and handle it better?