Would be nice if the doco at https://n8n.io/workflows/8 included the parameters etc so this could be used as a reference when people wish to write their own custom error handler.
I know read code is also a valid answer but in the spirit of completeness it should be here.
That page is less for documentation more for reference and to show some examples. If you want to know the available parameters there are multiple ways:
You can create an Error-Trigger Node and press “Execute Workflow”. It will then load some default error data that you can simply select it with the Expression-Editor
Think “read the code” could be a valid answer for some projects but as we aim to be usable by everybody also non-programmers (sadly not 100% there yet) would it not be one for us.