Downgrade identity server failure to FAIL_PROMPT instead of FAIL_ERROR

Spec for https://github.com/matrix-org/matrix-doc/pull/2284

content/client-server-api/_index.md

@@ -299,7 +299,7 @@ specify parameter values. The flow for this method is as follows:
         Validation for this URL is done as in the step above, but using
         `/_matrix/identity/api/v1` as the endpoint to connect to. If the
         `m.identity_server` property is present, but does not have a
-        `base_url` value, then `FAIL_ERROR`.
+        `base_url` value, then `FAIL_PROMPT`.
 
 {{% http-api spec="client-server" api="wellknown" %}}