Validate if /preview is running inside an Expo project

[byCedric]

Description of the feature

If you omit/forget to add the working-directory, you'll get a warning like: expo/eas-cli#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