check that yarn is installed and print a useful error message if not

Makefile

@@ -21,7 +21,7 @@ all: gitlab-setup gitlab-shell-setup gitlab-workhorse-setup support-setup gitaly
 # Set up the GitLab Rails app
-gitlab-setup: gitlab/.git gitlab-config bundler .gitlab-bundle .gitlab-yarn
+gitlab-setup: gitlab/.git gitlab-config bundler .gitlab-bundle yarn .gitlab-yarn
 gitlab/.git:
 	git clone ${gitlab_repo} gitlab
@@ -59,6 +59,14 @@ gitlab/public/uploads:
 bundler:
 	command -v $@ > /dev/null || gem install $@
+.PHONY:	yarn
+yarn:
+	@command -v $@ > /dev/null || {\
+		echo "Error: Yarn executable was not detected in the system.";\
+		echo "Download Yarn at https://yarnpkg.com/en/docs/install";\
+		exit 1;\
+	}
+
 # Set up gitlab-shell
 gitlab-shell-setup: symlink-gitlab-shell ${gitlab_shell_clone_dir}/.git gitlab-shell/config.yml bundler .gitlab-shell-bundle gitlab-shell/.gitlab_shell_secret