downloader.rb: fix exception arguments

* tool/downloader.rb (Downloader.download): fix the exception to
  re-raise.  initialize methods of subclasses of Exception may
  have different parameters.  [ruby-core:67086] [Bug #10639]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48963 b2dd03c8-39d4-4d8f-98ff-823fe69b080e