ERROR_REPO_DOWNLOAD_FAILED should have "Try Again" button

[Matthew Paul Thomas]

<https://wiki.ubuntu.com/SoftwarePackageOperations#repo-failed>:

"If downloading the package list fails (ERROR_REPO_DOWNLOAD_FAILED), then:

"1. If the computer is no longer connected to the Internet, an error alert should appear, with primary text “The software catalog can’t be updated because there is no Internet connection.”, secondary text “Connect to the Internet and try again.”, and buttons “Cancel” and “Try Again”. If an Internet connection begins while the alert is open, it should close itself and the download should start again automatically.

"2. Otherwise, an error alert should appear with primary text “The software catalog can’t be updated right now.”, secondary text any error from apt, and buttons “Cancel” and “Try Again”."