Return ErrClosed on Add() when the watcher is closed (#516)

Return ErrClosed on Add() when the watcher is closed (#516)

Also consistently return nil for Remove() and WatchList(). For Remove()
we could return an error, but I don't really see the point: if the
watcher is closed it's basically a no-op so returning nil is fine.

For WatchList() I'd like to return an error, but there is no error
return, so explicitly returning nil is the best we can do.

Fixes #511