ignore invalid json on error pages

src/matrix/net/request/fetch.js

@@ -85,7 +85,15 @@ export function createFetchRequest(createTimeout) {
         }
         const promise = fetch(url, options).then(async response => {
             const {status} = response;
-            const body = await response.json();
+            let body;
+            try {
+                body = await response.json();
+            } catch (err) {
+                // some error pages return html instead of json, ignore error
+                if (!(err.name === "SyntaxError" && status >= 400)) {
+                    throw err;
+                }
+            }
             return {status, body};
         }, err => {
             if (err.name === "AbortError") {

src/matrix/net/request/xhr.js

@@ -66,7 +66,7 @@ export function xhrRequest(url, options) {
     const xhr = send(url, options);
     const promise = xhrAsPromise(xhr, options.method, url).then(xhr => {
         const {status} = xhr;
-        let body = xhr.responseText;
+        let body = null;
         if (xhr.getResponseHeader("Content-Type") === "application/json") {
             body = JSON.parse(body);
         }