chore: adjust yarn warning [ci skip]

.github/contributing.md

@@ -39,7 +39,7 @@ Hi! I'm really excited that you are interested in contributing to Vue.js. Before
 
 ## Development Setup
 
-You will need [Node.js](http://nodejs.org) **version 10+**, and [Yarn](https://yarnpkg.com/en/docs/install).
+You will need [Node.js](http://nodejs.org) **version 10+**, and [Yarn 1.x](https://yarnpkg.com/en/docs/install).
 
 After cloning the repo, run:
 

scripts/checkYarn.js

@@ -1,5 +1,6 @@
 if (!/yarn\.js$/.test(process.env.npm_execpath || '')) {
   console.warn(
-    "\u001b[33mYou don't seem to be using yarn. This could produce unexpected results.\u001b[39m"
+    '\u001b[33mThis repository requires Yarn 1.x for scripts to work properly.\u001b[39m\n'
   )
+  process.exit(1)
 }