"Could not find watchman" doesn't actually mean it can't find watchman!

I've been doing an ember-cli upgrade, and I started seeing this error message when I start the server:

$ ember serve
Could not find watchman, falling back to NodeWatcher for file system events.
Visit http://www.ember-cli.com/user-guide/#watchman for more info.

This was frustrating because I knew I had watchman installed just fine!

$ which watchman
/usr/local/bin/watchman

This issue on the watchman repo was helpful in figuring this out. Running watchman watch . gave a much more interesting error message:

$ watchman watch .
/Users/benji/Library/LaunchAgents/com.github.facebook.watchman.plist: Operation already in progress ^C ```

The solution for me was to tell launchtl to forget about watchman:

$ launchctl unload ~/Library/LaunchAgents/com.github.facebook.watchman.plist 

After that, ember-cli ran watchman just fine.

There's an issue for this; I'm going to post a fix to clarify the language of the error.