macOS watchman requirement easy to miss and hard to figure out

I upgraded my Mac Book and wound myself around the axle for a good hour or so trying to debug why `ember serve` was failing with EMFILE too many file handles. The build would then seem to hang for 30s after an initial error and then would proceed, finally failing several seconds later.

`DEBUG=ember-cli:watcher ember s` finally revealed the issue: watchman was not installed, but it took a while to get there.

I suggest a quick check be done up front to issue a warning at launch if watchman is not present as I presume the failure could happen anywhere.