`init` command has runtime errors that should be unit tests

[grabbou]

There's no need to validate React Native's package.json on client-side. This should be an error before we even publish React Native (or removed at all as it sounds like a fundamental feature of React Native).

Throwing this during runtime has no point.

https://github.com/react-native-community/react-native-cli/blob/a56441021f6455b61f353fcb137d17c86ccc0043/packages/cli/src/commands/init/init.js#L54-L67