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>

config/boot.rb

+unless ENV.key?('RAILS_ENV')
+  STDERR.puts 'ERROR: Missing RAILS_ENV environment variable, please set it to "production", "development", or "test".'
+  exit 1
+end
+
 ENV['BUNDLE_GEMFILE'] ||= File.expand_path('../Gemfile', __dir__)
 
 require 'bundler/setup' # Set up gems listed in the Gemfile.