1) why are both errors in this case being triggered?
2) is there any case where this error is included where it's not fatal to the request? (since the docs don't link the possible error states for the mutation I don't know how to look this up)
Is there any documentation specifically related to this error?
Is there a prefered way to handle this error? Should I only look for userError, or do I need to look for checkoutUserError in specific cases?
How can I know that checkoutUserError seems to inherit from userError in some way?
I also cannot find a list of possible error states anywhere in the docs- for example is there a page that mentions "Checkout does not exist" specifically? Shouldn't I be able to find this in the doc pages above?