registry.rb: try en_US message

* ext/win32/lib/win32/registry.rb (Win32::Registry::Error#initialize):
  try en_US message if the default message cannot be encoded to
  locale.  [ruby-core:65295] [Bug #10300]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48927 b2dd03c8-39d4-4d8f-98ff-823fe69b080e