Add clean error message when RAILS_ENV is unset (#15381)

Add clean error message when RAILS_ENV is unset (#15381)

By default, an unset RAILS_ENV is understood as RAILS_ENV=development.

However, this is not what most people expect, and due to development-only
dependencies, users are often left with confusing error messages.

This commit changes it so that an explicit RAILS_ENV is required, and
failing that, an error message is displayed before loading the app.

Co-authored-by: Claire <claire.github-309c@sitedethib.com>