Silence LoadError only if it is for `rubygems` itself

Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>
Co-authored-by: Hiroshi SHIBATA <hsbt@ruby-lang.org>

gem_prelude.rb

@@ -1,6 +1,8 @@
 begin
   require 'rubygems'
-rescue LoadError
+rescue LoadError => e
+  raise unless e.path == 'rubygems'
+
   warn "`RubyGems' were not loaded."
 end if defined?(Gem)