I’ve got some migrations and they are basically doing their job. My problem is that errors in the data source end up in a WSOD in Migrate Tools UI, without any human readable error message (when I click on “List migration”).

E.g. my source usually responds something like

{success: true, result: {my_dataset: {data1: 'Alice', data2: 'Bob'}}}

and in my YML I’ve got something like

source:   plugin: url   data_parser_plugin: json   item_selector: /result/my_dataset   fields:     -       name: data1       label: 'Just an example'       selector: data1 

Today my source went down and returns a HTTP 500 with

{success: false, message: 'We are working on it, try later...'}

How can I force the Migrate Tools UI to display an error message and stay in the admin theme, instead of a WSOD?