Fix exception when download file doesn't exist
Fix exception when download file doesn't exist

If the package.json from the manifestUrl exists, and the app needs an
update, but the file from the update/download link doesn't exist node
throws an exception and dies. This allows the script to continue.

p5150j/node-webkit-updater,edjafarov/node-webkit-updater,btjones/node-webkit-updater,p5150j/node-webkit-updater,firsara/node-webkit-updater,yufi113/node-webkit-updater,innerverse/node-webkit-updater,yufi113/node-webkit-updater,btjones/node-webkit-updater,innerverse/node-webkit-updater,edjafarov/node-webkit-updater,firsara/node-webkit-updater