renderAsync can throw synchronously, should return a rejected promise instead?

[tomyam1]

This can happen, for instance, if the underlying transformer only supports render.

It will throw synchronously when calling it.

A simple solution would be to wrap all calls to the jstransformer in a try-catch.

What do you think?

More thoughts:

1. It's bad that renderAsync can do that because other modules don't expect an async function to throw synchronously, e.g. (fix) add filename when jstransformer throws synchronously metalsmith/in-place#174

2. It's not an issue with the underlying jstransformer because it supports render which can throw synchronously.

[ForbesLindesay]

Yes, we should wrap in a try catch to handle synchronous errors. I'd be happy to accept a pull request for this.