Improved user errors on the fulfillmentOrderCancel mutation

The fulfillmentOrderCancel mutation now returns a specific fulfillmentOrderCancelError type instead of the generic userError type.

What's changing:

The error response now includes a code field for easier programmatic error handling and identification.

Why we made this change:

This enhancement supports the new feature for manually marking fulfillment orders as in-progress, which allows merchants to self-report progress on their fulfillment orders. As part of this feature, cancelling fulfillment orders with reported progress is now restricted, and the enhanced error responses help developers handle these restrictions more effectively.