Describe the bug

When an error occurs while loading the postcss config it is swallowed:

https://github.com/vuejs/vite/blob/79a55b5d5fe11316b99d09d1ce0e357fa3335ac4/src/node/utils/resolvePostCssConfig.ts#L16

I'd be happy to file a PR, that will fail the build in production mode and return a 500 for the css in dev mode. Any concerns?

Reproduction

n/a

System Info

• required vite version: 0.14.1
• required Operating System: OS X
• required Node version: 12

Logs (Optional if provided reproduction)

none

Can you provider repo for this?