Improve error handling of Webpack manifest loading

If an HTTP error occurs loading the manifest, we saw an obscure JSON
parse error. Now we look for the 200 response code and raise an error if
we don't get back a valid response.

lib/gitlab/webpack/manifest.rb

@@ -90,12 +90,17 @@ module Gitlab
         def load_dev_server_manifest
           host = ::Rails.configuration.webpack.dev_server.host
           port = ::Rails.configuration.webpack.dev_server.port
-          http = Net::HTTP.new(host, port)
-          http.use_ssl = ::Rails.configuration.webpack.dev_server.https
-          http.verify_mode = OpenSSL::SSL::VERIFY_NONE
-          http.get(dev_server_path).body
+          scheme = ::Rails.configuration.webpack.dev_server.https ? 'https' : 'http'
+          uri = Addressable::URI.new(scheme: scheme, host: host, port: port, path: dev_server_path)
+
+          # localhost could be blocked via Gitlab::HTTP
+          response = HTTParty.get(uri.to_s, verify: false) # rubocop:disable Gitlab/HTTParty
+
+          return response.body if response.code == 200
+
+          raise "HTTP error #{response.code}"
         rescue => e
-          raise ManifestLoadError.new("Could not load manifest from webpack-dev-server at http://#{host}:#{port}#{dev_server_path} - is it running, and is stats-webpack-plugin loaded?", e)
+          raise ManifestLoadError.new("Could not load manifest from webpack-dev-server at #{uri} - is it running, and is stats-webpack-plugin loaded?", e)
         end
 
         def load_static_manifest