feat(watcher): use polling on Mac

Revert to polling on Mac. Use OS events on Linux and Windows.

Eventually I wanna get rid off polling even on Mac, however it's not ready yet.

See:
https://github.com/pipobscure/fsevents/issues/17
https://github.com/paulmillr/chokidar/issues/87

Basically reverts e88fbc24dd34e7976cae2547bad07e6f044a768b.
fix(client): show error if an adapter is removed

Before we would only show the error when this happens in the first run.
It's not likely to happen, but if the adapter is removed in a subsequent run, it would throw "null is not a function".