Auto-fill @throws

[KittyGiraudel]

Now that Sass 3.4 has an @error directive, it's easier than ever to auto-fill @throws by fetching all the @error and their error message.

Thoughts?

[FWeinb]

As mentioned in #232 I will extend the annotation plugin structure to do these things in a more performant way. This can be done in 1.10