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.