Show warning message about binstub outside generation.

  Fixes rubygems#6149.

lib/bundler/templates/Executable

@@ -12,8 +12,15 @@ require "pathname"
 ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../<%= relative_gemfile_path %>",
   Pathname.new(__FILE__).realpath)
 
+def bundle_stub?(path)
+    return false unless File.file?(path)
+    return true if File.read(path, 150) =~ /This file was generated by Bundler/
+    abort("Your `bin/bundle` was not generated by Bundler, so this binstub cannot run.
+Replace `bin/bundle` by running `bundle binstubs bundler --force`, then run this command again.")
+end
+
 bundle_binstub = File.expand_path("../bundle", __FILE__)
-load(bundle_binstub) if File.file?(bundle_binstub)
+load(bundle_binstub) if bundle_stub?(bundle_binstub)
 
 require "rubygems"
 require "bundler/setup"