The summary read degraded for 15 minutes after any single node_error, even
once the node had run well again and including one bad input from a visitor.
The controller now counts a node's failures since it last ran well and notes
when it recovered; both carry across rebuilds and an acknowledgement resets
the count. Two in a row inside the 15 minute window is "failing" (a problem),
one is listed under warnings, and nodes.failing keeps meaning "failing now".
