display the currently set FLoC header in the checkup page's warning message.

Addresses #757

www/checkup/main.js

@@ -391,21 +391,32 @@ define([
 
     assert(function (cb, msg) {
         setWarningClass(msg);
-        msg.appendChild(h('span', [
-            "You haven't opted out of participation in Google's ",
-            code('FLoC'),
-            " targeted advertizing network. This can be done by adding a ",
-            code('permissions-policy'),
-            " HTTP header with a value of ",
-            code('interest-cohort=()'),
-            " in your reverse proxy's configuration. See the provided NGINX configuration file for an example. ",
-            h('p', [
-                link("https://www.eff.org/deeplinks/2021/04/am-i-floced-launch", 'Learn more'),
-            ]),
-        ]));
+
+        var printMessage = function (value) {
+            msg.appendChild(h('span', [
+                "This instance hasn't opted out of participation in Google's ",
+                code('FLoC'),
+                " targeted advertizing network. ",
+
+                "This can be done by setting a ",
+                code('permissions-policy'),
+                " HTTP header with a value of ",
+                code('"interest-cohort=()"'),
+                " in the configuration of its reverse proxy instead of the current value (",
+                code(value),
+                "). See the provided NGINX configuration file for an example. ",
+
+                h('p', [
+                    link("https://www.eff.org/deeplinks/2021/04/am-i-floced-launch", 'Learn more'),
+                ]),
+            ]));
+        };
+
         $.ajax('/?'+ (+new Date()), {
             complete: function (xhr) {
-                cb(xhr.getResponseHeader('permissions-policy') === 'interest-cohort=()');
+                var header = xhr.getResponseHeader('permissions-policy');
+                printMessage(JSON.stringify(header));
+                cb(header === 'interest-cohort=()' || header);
             },
         });
     });
@@ -735,11 +746,18 @@ define([
     };
 
     var failureReport = function (obj) {
+        var printableValue = obj.output;
+        try {
+            printableValue = JSON.stringify(obj.output);
+        } catch (err) {
+            console.error(err);
+        }
+
         return h('div.error', [
             h('h5', obj.message),
             h('table', [
                 row(["Failed test number", obj.test + 1]),
-                row(["Returned value", obj.output]),
+                row(["Returned value", code(printableValue)]),
             ]),
         ]);
     };