Use abort instead of warn(); exit in boot.rb env check (#29209)

config/boot.rb

@@ -1,8 +1,15 @@
 # frozen_string_literal: true
 
 unless ENV.key?('RAILS_ENV')
-  warn 'ERROR: Missing RAILS_ENV environment variable, please set it to "production", "development", or "test".'
-  exit 1
+  abort <<~ERROR
+    The RAILS_ENV environment variable is not set.
+
+    Please set it correctly depending on context:
+
+      - Use "production" for a live deployment of the application
+      - Use "development" for local feature work
+      - Use "test" when running the automated spec suite
+  ERROR
 end
 
 ENV['BUNDLE_GEMFILE'] ||= File.expand_path('../Gemfile', __dir__)