wkwebview doesn't work in runghc on macOS Mojave

[xplat]

MacOS has a notion of 'main thread' and in runghc a different thread is being used as the UI thread. On Mojave this no longer works -- you get a big WARNING on stderr, but jsaddle seems to think everything succeeded and keeps running. Unfortunately, the only UI element that comes up is a dock icon.

There may not be a way to make runghc and other things that use a spawned thread as a UI thread work on this version of macOS, but at least they should fail cleanly.