Relax identity server discovery checks to FAIL_PROMPT

As discussed in MSC2284, this relaxes the identity server discovery to a
`FAIL_PROMPT` state so that clients can choose to warn and continue.

Part of element-hq/element-web#11102
Implements matrix-org/matrix-spec-proposals#2284

src/autodiscovery.js

@@ -1,5 +1,6 @@
 /*
 Copyright 2018 New Vector Ltd
+Copyright 2019 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -275,21 +276,11 @@ export class AutoDiscovery {
         let isUrl = "";
         if (wellknown["m.identity_server"]) {
             // We prepare a failing identity server response to save lines later
-            // in this branch. Note that we also fail the homeserver check in the
-            // object because according to the spec we're supposed to FAIL_ERROR
-            // if *anything* goes wrong with the IS validation, including invalid
-            // format. This means we're supposed to stop discovery completely.
+            // in this branch.
             const failingClientConfig = {
-                "m.homeserver": {
-                    state: AutoDiscovery.FAIL_ERROR,
-                    error: AutoDiscovery.ERROR_INVALID_IS,
-
-                    // We'll provide the base_url that was previously valid for
-                    // debugging purposes.
-                    base_url: clientConfig["m.homeserver"].base_url,
-                },
+                "m.homeserver": clientConfig["m.homeserver"],
                 "m.identity_server": {
-                    state: AutoDiscovery.FAIL_ERROR,
+                    state: AutoDiscovery.FAIL_PROMPT,
                     error: AutoDiscovery.ERROR_INVALID_IS,
                     base_url: null,
                 },