Remove duplicate writeError.

Both the livereload server and the watcher were listening to the `watcher`'s `error` event which caused the same error to be printed twice.

The `didError` in `livereload-server` is superfluous (it cannot run without `wacther` running and `watcher` already has a `didError` that does the right thing).

Copies tests added in #2246, into the watcher models tests.