Fix Javascript issue on non sqlite databases

When a non sqlite database is used, loading the admin interface fails
because the backup button is not generated.
This PR is solves it by checking if the elements are valid.

Also made some other changes and fixed some eslint errors.
Showing `_post` errors is better now.

Update jquery to latest version.

Fixes #3166

src/static/scripts/admin.js

@@ -1,4 +1,6 @@
 "use strict";
+/* eslint-env es2017, browser */
+/* exported BASE_URL, _post */
 
 function getBaseUrl() {
     // If the base URL is `https://vaultwarden.example.com/base/path/`,
@@ -26,6 +28,8 @@ function msg(text, reload_page = true) {
 }
 
 function _post(url, successMsg, errMsg, body, reload_page = true) {
+    let respStatus;
+    let respStatusText;
     fetch(url, {
         method: "POST",
         body: body,
@@ -33,22 +37,30 @@ function _post(url, successMsg, errMsg, body, reload_page = true) {
         credentials: "same-origin",
         headers: { "Content-Type": "application/json" }
     }).then( resp => {
-        if (resp.ok) { msg(successMsg, reload_page); return Promise.reject({error: false}); }
-        const respStatus = resp.status;
-        const respStatusText = resp.statusText;
+        if (resp.ok) {
+            msg(successMsg, reload_page);
+            // Abuse the catch handler by setting error to false and continue
+            return Promise.reject({error: false});
+        }
+        respStatus = resp.status;
+        respStatusText = resp.statusText;
         return resp.text();
     }).then( respText => {
         try {
             const respJson = JSON.parse(respText);
-            return respJson ? respJson.ErrorModel.Message : "Unknown error";
+            if (respJson.ErrorModel && respJson.ErrorModel.Message) {
+                return respJson.ErrorModel.Message;
+            } else {
+                return Promise.reject({body:`${respStatus} - ${respStatusText}\n\nUnknown error`, error: true});
+            }
         } catch (e) {
-            return Promise.reject({body:respStatus + " - " + respStatusText, error: true});
+            return Promise.reject({body:`${respStatus} - ${respStatusText}\n\n[Catch] ${e}`, error: true});
         }
     }).then( apiMsg => {
-        msg(errMsg + "\n" + apiMsg, reload_page);
+        msg(`${errMsg}\n${apiMsg}`, reload_page);
     }).catch( e => {
         if (e.error === false) { return true; }
-        else { msg(errMsg + "\n" + e.body, reload_page); }
+        else { msg(`${errMsg}\n${e.body}`, reload_page); }
     });
 }