Validate if `/preview` is running inside an Expo project

### Description of the feature
If you omit/forget to add the `working-directory`, you'll get a warning like: https://github.com/expo/eas-cli/issues/1779. It's sub-optimal and does not point users to the right fix. We should probably validate if it's running correctly inside an Expo app, and if not, provide a better error message.

### Motivation
See above