Handle download errors gracefully

[abetusk]

When downloading projects, try and handle the downloads more gracefully. If an error occurred, do one/some/all of:

• report that error to the user
• provide what you have as a packaged downloadable file
• provide an error message inside of the downloaded file
• send an email notification (maybe rate limited) so that the administrator/operator knows a critical error occurred.

estimate: 3 priority: high