Rails config for raise on missing translations

If I recall correctly, it disappeared from `3-0` and was missing ever since, restored back in `4-1`. This PR backports @kassio's [commit](https://github.com/rails/rails/commit/433628a45c2f5dd04b115af1b5579dac75255c67) from `4-1` to `4-0`.

Would be great if we could pack this in `4.0.6` release.

/cc @tamird