Display proper error when specified plugin isn't found (#3672)

Picking up on https://github.com/nuxt/nuxt.js/pull/3434 -- synced with dev and tests included.

lib/builder/builder.js

@@ -442,6 +442,13 @@ export default class Builder {
       compilersOptions.push(serverConfig)
     }
 
+    // Check plugins exist then set alias to their real path
+    await Promise.all(this.plugins.map(async (p) => {
+      if (!await fsExtra.pathExists(p.src)) {
+        throw new Error(`Plugin not found: ${r(p.src)}`)
+      }
+    }))
+
     // Alias plugins to their real path
     this.plugins.forEach((p) => {
       const src = this.relativeToBuild(p.src)

test/fixtures/missing-plugin/nuxt.config.js

@@ -0,0 +1,3 @@
+export default {
+  plugins: ['~/plugins/missing.js']
+}

test/unit/nuxt.test.js

@@ -43,4 +43,16 @@ describe('nuxt', () => {
 
     await nuxt.close()
   })
+
+  test('Fail to build when specified plugin isn\'t found', () => {
+    const nuxt = new Nuxt({
+      dev: false,
+      rootDir: resolve(__dirname, '..', 'fixtures', 'missing-plugin')
+    })
+
+    return new Builder(nuxt).build().catch((err) => {
+      const s = String(err)
+      expect(s.includes('Plugin not found')).toBe(true)
+    })
+  })
 })