exit code 1 in case of failed builds (#3535)

Fixes https://github.com/nuxt/nuxt.js/issues/3534

lib/builder/builder.js

@@ -525,6 +525,10 @@ export default class Builder {
             if (this.options.test) {
               err = stats.toString(this.options.build.stats)
             }
+            if (!err) {
+              // actual errors will be printed by webpack itself
+              err = 'Nuxt Build Error'
+            }
 
             return reject(err)
           }