fix: better error message for missing key entry in pages config

fixes #2816

packages/@vue/cli-service/lib/options.js

@@ -11,7 +11,9 @@ const schema = createSchema(joi => joi.object({
   productionSourceMap: joi.boolean(),
   parallel: joi.boolean(),
   devServer: joi.object(),
-  pages: joi.object(),
+  pages: joi.object().pattern(/\w+/, joi.object().keys({
+    entry: joi.string().required()
+  }).unknown(true)),
   crossorigin: joi.string().valid(['', 'anonymous', 'use-credentials']),
   integrity: joi.boolean(),