Improve Error Handling at FatalErrorBoundary

[thedavidprice]

Fixes #140

Adds the Webpack Plugin 'ErrorOverlayPlugin':
https://github.com/smooth-code/error-overlay-webpack-plugin

Error display before:

Something went wrong

Error display after:

[mojombo]

Nice! What’s the behavior in production? We need to ensure this info doesn’t leak in that environment.

[thedavidprice]

The plugin states by default it's "development ENV only" but I'd like to test it. I first wanted to see if Peter agrees with this plugin/implementation, but your question was also on my mind.

If this is the path forward, I’ll make sure we test in a Production environment before releasing it.

[peterp]

Nice! Tom makes a good point, maybe we should only import it in core/webpack.development.js?

Other than that the implementation is great.

[thedavidprice]

@peterp moved settings to webpack dev config. Please go ahead and merge if it all's good.