Hi there,
I wish I had kept the message, but essentially it was that I couldn't delete the manufacturer categories and that the process had failed.
I was cleaning out some sample data from my install and it took me a moment or two to work out why I couldn't delete them. Once I had gone and deleted all the products (hence the manufacturer categories would now be empty) I was then able to delete those categories.
What I would suggest is a replacement error message such as "Manufacturer categories can't be deleted while products are assigned to them."
Many thanks.
Andrew