Bound the read of bus-module events, keep S9 seeing when it fails, count the controller's own slow consumer once (hq issue 270, review of #326)

A storm of closures must not spend a watchdog tick, so the read stops at 2000
events and says the count is a floor. A failed read is its own condition
rather than blinding S9, so the controller's own signals are still said. The
controller's fall behind, told to its client and closed by the server, is one
fault, counted once.

cmd/mesh-controller/plain_words.go

@@ -620,6 +620,12 @@ var plainWordings = map[string]func(conditions.Observation) words{
 			Explanation: "The bus reports a listener too slow to keep up, so messages to it are late.",
 			Resolved:    "The listener keeps up again"}
 	}),
+	kindBusFaultsUnread: worded(func(o conditions.Observation) words {
+		return words{Headline: "Slow clients of the bus cannot be checked",
+			Explanation: "The controller could not read what the bus said about its clients' connections, so a " +
+				"client the bus cut off would go unnoticed. It tries again at every check.",
+			Resolved: "Resolved: the bus's clients are checked again"}
+	}),
 	"connection-closed": worded(func(o conditions.Observation) words {
 		return words{Headline: "The bus cut a client off",
 			Explanation: "The bus closed a client's connection because it broke the bus's rules, so what it was " +