Raise RuntimeError when gist is unavailable

plugins/gist_tag.rb

@@ -82,6 +82,9 @@ module Jekyll
       https.verify_mode = OpenSSL::SSL::VERIFY_NONE
       request           = Net::HTTP::Get.new raw_uri.request_uri
       data              = https.request request
+      if data.code != 200
+          raise RuntimeError, "Gist replied with #{data.code} for #{gist_url}"
+      end
       data              = data.body
       cache gist, file, data unless @cache_disabled
       data