Exit production build if any errors are in build stats

Created by: pdillon

Until https://github.com/webpack/webpack/issues/2390 is addressed and all errors trigger the --bail switch, would be helpful to manually surface them from the production build stats passed in the callback.

Sample output

> react-scripts build

Creating an optimized production build...
Error(s) occurred during the production build:
static/js/main.9a6b731a.js from UglifyJs
SyntaxError: Unexpected token: name .....