Consolidated missing endpoint error detection in Messages.

src/content/content.js

@@ -62,7 +62,7 @@ var notifyPage = async () => {
       return true;
     } catch (e) {
       debug(e);
-      if (/Receiving end does not exist/.test(e.message)) {
+      if (Messages.isMissingEndpoint(e)) {
         window.setTimeout(notifyPage, 2000);
       }
     }

src/content/staticNS.js

@@ -49,7 +49,7 @@
             {id: "fetchPolicy", url, contextUrl: url});
           break;
         } catch (e) {
-          if (e.message !== "Could not esablish connection. Receiving end does not exist.") {
+          if (!Messages.isMissingEndpoint(e)) {
             break;
           }
           error("Background page ready yet, retrying to fetch policy...")

src/lib/Messages.js

@@ -65,6 +65,10 @@
         return await browser.tabs.sendMessage(recipientInfo.tabId, args, opts);
       }
       return await browser.runtime.sendMessage(args);
+    },
+    isMissingEndpoint(error) {
+      return error && error.message ===
+        "Could not esablish connection. Receiving end does not exist.";
     }
   }
 }