[JENKINS-6798] Redirect on HTTP 502, 503, 504 (#4366)

* [JENKINS-6798] Redirect on HTTP >= 502 (502, 503, 504)

When Jenkins is behind a reverse proxy and is not available or
restarting, the reverse proxy will answer a HTTP 502 (because the upstream
doesn't answer).

* Add upper bound

war/src/main/webapp/scripts/hudson-behavior.js

@@ -2778,7 +2778,7 @@ function applySafeRedirector(url) {
         new Ajax.Request(url, {
             method: "get",
             onFailure: function(rsp) {
-                if(rsp.status==503 && rsp.getHeader("X-Jenkins-Interactive")==null) {
+                if((rsp.status >= 502 && rsp.status <= 504) && rsp.getHeader("X-Jenkins-Interactive")==null) {
                   // redirect as long as we are still loading
                   window.setTimeout(statusChecker,5000);
                 } else {

war/src/main/webapp/scripts/loading.js

@@ -47,7 +47,7 @@ function safeRedirector(url) {
     window.setTimeout(function() {
         const statusChecker = arguments.callee;
         Fetch.get(url, function(error, status) {
-            if(status === 503 || status === 0) {
+            if((status >= 502 && status <= 504) || status === 0) {
                 window.setTimeout(statusChecker, timeout)
             } else {
                 window.location.replace(url);