Exit when no gem is found; expand on error message

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@4114 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

railties/environments/boot.rb

@@ -24,7 +24,11 @@
     if rails_gem
       require rails_gem.full_gem_path + '/lib/initializer'
     else
-      STDERR.puts "Cannot find gem for Rails =#{RAILS_GEM_VERSION}. Install missing gem or change config/boot.rb."
+      STDERR.puts %(Cannot find gem for Rails =#{RAILS_GEM_VERSION}:
+  Install the missing gem with 'gem install -v=#{RAILS_GEM_VERSION} rails', or
+  change environment.rb to define RAILS_GEM_VERSION with your desired version.
+)
+      exit 1
     end
   else
     require 'initializer'