shell: Fix cockpit.Machines.Update() error handling

Stringify the error object, so that we don't just get an empty value in
our test logs.

This call requires superuser privileges to write into /etc/cockpit/, so
declare that and avoid making console noise if the session is
unprivileged. That way, we only see unexpected failures.

pkg/shell/machines/machines.js

@@ -220,15 +220,15 @@ function Machines() {
         }
 
         // FIXME: investigate re-using the proxy from Loader (runs in different frame/scope)
-        const bridge = cockpit.dbus(null, { bus: "internal", superuser: "try" });
+        const bridge = cockpit.dbus(null, { bus: "internal", superuser: "require" });
         const mod =
             bridge.call("/machines", "cockpit.Machines", "Update", ["99-webui.json", host, values_variant])
-                    .catch(function(error) {
-                        console.error("failed to call cockpit.Machines.Update(): ", error);
+                    .catch(error => {
+                        // avoid make noise when we are not superuser
+                        if (error.problem !== "access-denied")
+                            console.error("failed to call cockpit.Machines.Update(): ", JSON.stringify(error));
                     })
-                    .then(() => {
-                        self.overlay(host, values);
-                    });
+                    .then(() => self.overlay(host, values));
 
         return mod;
     }

test/common/testlib.py

@@ -1721,7 +1721,6 @@ def login_and_go(self, path: Optional[str] = None, user: Optional[str] = None, h
         "aria-label",
         # messages from our own pages
         "CPU temperature metric closed.*",
-        "failed to call cockpit.Machines.Update().*",
         "Getting properties for problem.*",
         "Channel for reportd D-Bus client closed.*",
     ]