How about a warning that says that it will not work. That you cannot fit a 500+GB blockchain in 400GB free space and that more then likely things are going to go poorly.
In the initial startup dialog, it does show that. If you don't have enough free space, there will be red warning text saying that there isn't enough free space.
Also, in master, there will be a warning dialog on first start if the disk the datadir is on doesn't have enough free space.