fix (firefox): reported error message

on firefox, whenever we were navigating away from the main page, it
would trigger the error handling code that would show the message: "Service unavailable, if the problem persist, contact your administrator" before the page change. This is only hapenning in Firefox but as a precautionary measure we're trying to detect those cases upfront so we wait instead of triggering the error handling

Author: mickael-kerjean

public/assets/lib/ajax.js

@@ -33,10 +33,19 @@ export default function(opts) {
             }
             return res;
         }),
-        rxjs.catchError((err) => rxjs.throwError(processError(err.xhr, err))),
+        rxjs.catchError(
+            (err) => activePage
+                ? rxjs.throwError(processError(err.xhr, err))
+                : rxjs.EMPTY
+        ),
     );
 }
 
+let activePage = true;
+window.addEventListener("beforeunload", function() {
+    activePage = false;
+});
+
 function parseDataUrl(url) {
     const matches = url.match(/^data:(.*?)(;base64)?,(.*)$/);
     if (!matches) throw new Error("Invalid Data URL");