Fix error handling when compiling CSS and JS

[thecrypticace]

I cloned the master branch because I wanted to start playing with the Webpack integration and discovered that the error handling was broken. So, here's a fix. :)

Aside 1: CSS minification appears to also be broken but I haven't figured that one out yet. See #527
Aside 2: Compiling requires babel-preset-es2015 but it's not in the devDependencies. Intentional?

[JeffreyWay]

Thanks!

We're using Buble for compilation, so that Babel preset won't be needed.

[thecrypticace]

Glad to help.

The compile script in package.json still uses babel which is why I asked.