Customizing the system errors is not the direction I would like to go.
Most common solution is to not use a microflow as datasource but prepare the data to show and associate then to the main object via a special association. Update that association on the right time, or before opening the form.
To make a sample. Customers and Orders. Make an extra reference set between Customers and Orders. (Customer_VisibleOrder). Fill the association before and add a datagrid over that association.