chore: should output meaningful error message for missing vue-jest dep

follow up of #6418

Author: haoqunjiang

Diff for: packages/@vue/cli-plugin-unit-jest/presets/default/jest-preset.js

@@ -1,3 +1,10 @@
+let vueJest = null
+try {
+  vueJest = require.resolve('vue-jest')
+} catch (e) {
+  throw new Error('Cannot resolve "vue-jest" module. Please make sure you have installed "vue-jest" as a dev dependency.')
+}
+
 module.exports = {
   moduleFileExtensions: [
     'js',
@@ -8,7 +15,7 @@ module.exports = {
   ],
   transform: {
     // process *.vue files with vue-jest
-    '^.+\\.vue$': require.resolve('vue-jest'),
+    '^.+\\.vue$': vueJest,
     '.+\\.(css|styl|less|sass|scss|jpg|jpeg|png|svg|gif|eot|otf|webp|ttf|woff|woff2|mp4|webm|wav|mp3|m4a|aac|oga)$':
     require.resolve('jest-transform-stub'),
     '^.+\\.jsx?$': require.resolve('babel-jest')