better error message with too old node. Fixes 2880 

https://bugs.freenetproject.org/view.php?id=2880 
This is the short description of the `announceDisabledTooOld` alert. It is [described as](https://github.com/freenet/fred/blob/c4560061fc71251c02bc9178a6a992879ef90c5d/src/freenet/node/useralerts/UserAlert.java#L32) "*Really* concise text of alert. Should be comfortably under a line even when translated into a verbose language. Will link to the full details."

This kind of clarification should be on the `announceDisabledTooOld` string. I'd also be interested in getting in something about how it can only download if it's connected to peers? Do they show as peers if they're UOMing though?