when no appcast is available the Stack updater throws an error

this can happen either due to network connection or when the appcast is removed from the current location -- or anything else that prevents the fetch in a reasonable time.

when that happens the check fails and generates an error that gets shown to the user via an oldscool sparkle fully alert window.

this should definitely be a silent warning in most cases or perhaps a warning when the time between last check has been longer than a specific amount.

in any case, the alert window should probably be updated a bit.