fix(master): say why a pinned pick fails on a protocol 2 agent (#9)

internal/master/hub/route.go

@@ -16,7 +16,8 @@ type candidate struct {
 
 // Route picks the connection a job for svc goes to, among agents that speak
 // at least protocol version minProtocol. It returns model.ErrAgentOffline or
-// model.ErrAgentTooOld when there is none.
+// model.ErrAgentTooOld (model.ErrAgentTooOldForPinned when minProtocol asks
+// for a pinned ref) when there is none.
 func (h *Hub) Route(svc model.Service, minProtocol int) (model.Route, error) {
 	h.mu.Lock()
 	conns := make([]candidate, 0, len(h.conns))
@@ -64,6 +65,9 @@ func route(svc model.Service, conns []candidate, minProtocol int) (model.Route,
 		}
 	}
 	if tooOld {
+		if minProtocol >= protocol.MinPinnedRefVersion {
+			return model.Route{}, model.ErrAgentTooOldForPinned
+		}
 		return model.Route{}, model.ErrAgentTooOld
 	}
 	return model.Route{}, model.ErrAgentOffline