succd: Hint at invalid process values

When the succbone connection breaks, add hints to the UI that values may
no longer be correct.

succbone/succd/index.html

@@ -410,6 +410,15 @@ window.addEventListener("load", (_) => {
         socket.addEventListener("close", (event) => {
             status.innerHTML = "Offline";
             status.style = colors.highlightFault;
+
+            // Indicate all process values as unknown
+
+            [failsafe, highpressure, rp, dp, trough, thigh, volts, mbar].forEach((el) => {
+                if (!el.innerHTML.includes("??")) {
+                    el.innerHTML += "??";
+                }
+            });
+
             if (connected) {
                 console.log("Socket dead, reconnecting...");
             }