webpack build to emit errors in sane way

[maraisr]

Be very cool if webpack could break the build if --validate would for instance fail, as in do derived files exist or not. This would be a client-side error emitted in the loader in mode: development and a blatant break-the-build in mode: production.

In the same was a Relay emits the if (__DEV__ && hash === computedHash) { throw new Error(); } during its dev-mode.

[theKashey]

Now it can emit errors or autogenerate missing image
✅ in v1.1.1