Add specific error message when there is no default export for a Component

[mfickett]

What problem does this feature solve?

When a component .vue file does not have a default export, the Component cannot be loaded (as in #1234). Currently, the error message is unclear (I got "Uncaught TypeError: Cannot set property 'render' of undefined"). It would be clearer to generate an error saying explicitly that the component definition did not have a default export which is required.

What does the proposed API look like?

Error: Component MyComponent.vue does not have a default export. See https://vuejs.org/v2/guide/components-registration.html#Local-Registration .