Leaks suck, they at best creates a completely unoptimized map and more often than not result in very nasty visual errors. As such you probably don't even want to bother with vvis, vrad and starting the game if the map leaked. Especially not if you were going to do a time consuming final build. There is a trick that you can do to stop the build process if the map leaks. You see, when the map leaks vbsp creates no *.prt file. So if you can detect the lack of this file you can halt compilation. Note that areaportal leaks does not prevent this file from being created and as such can not be detected this way. Thankfully the advanced version of the run map dialog in Hammer allows you to check if a file has been created or not after each build step. This is perfect for this purpose. But there is a slight flaw, it can only check for one file, but vbsp is supposed to create two, both the *.bsp and the *.prt file. If you want to check for the existence of both you need to use multiple steps. What I do is that I add a dummy step that does nothing after vbsp and add in a check for the second file. The dummy step I use is "copy nul nul".