Better error handling for file uploads

[fosterfarrell9]

Currently, we have

client_max_body_size 4G;

in our config in docker/nginx.tmpl. A user tried to upload a 6GB video which conseqently failed. While I am not overly enthusiastic to rasing this to, say 8 GB, for resource reasons, maybe we should raise. Of course, I would alway encourage users to user better compression instead. Also, we might stop uploads of bigger files immediately with an error-message (as is done in the case of homework uploads). In the case above, the error message only came after the limit was reached during uploading.

[Splines]

These are really big files. If we allow a limit of 8GB, our SSDs will fill up pretty quickly for sure. Instead, we should implement a better error message in the frontend when users try to upload such a big file. Instead, they should compress their videos on their own before uploading. (Maybe in the future, we can even implement a compression service with ffmpeg or the like on our side, but that's a bigger todo I guess).

[fosterfarrell9]

I agree (and the corresponding user could compress his file to 1.4GB which worked fine). I changed the title of the issue in the direction that it would still be good to produce an error message immediately when too large files are uploaded.