Log error if settings unavailable when saving user settings

Fixes #1645

red/runtime/locales/en-US/runtime.json

@@ -75,6 +75,7 @@
     },
 
     "settings": {
+        "user-not-available": "Cannot save user settings: __message__",
         "not-available": "Settings not available",
         "property-read-only": "Property '__prop__' is read-only"
     },

red/runtime/settings.js

@@ -173,6 +173,9 @@ var persistentSettings = {
         return clone(userSettings[username]);
     },
     setUserSettings: function(username,settings) {
+        if (globalSettings === null) {
+            throw new Error(log._("settings.not-available"));
+        }
         var current = userSettings[username];
         userSettings[username] = settings;
         try {