Add custom information to error message

I would like to generate a new custom error message if a user attempts to delete a file or folder that they are not allowed due to that file/folder being referenced by something else. If the file/folder is being referenced by something else, then I would like to include in the error message the name of the thing that is referencing it so that the user knows how to resolve the issue and move forward.

I understand I can modify the DeleteFileCommandHandler and DeleteFolderCommandHandler to implement the new logic and throw a new error, but how can I propogate that error information from here down to the client where the user can see it?