Validate SKU before leaving product inventory

[nbradbury]

In #3560 we removed the Done button from the product detail sub-screens. This has a side effect of potentially not validating the SKU in the inventory screen.

If you enter a SKU that's already known remotely and quickly exit the dialog, it'll let you save the value but it'll give you an error saving the product.

[shiki]

Possibly another thing we can do is to display a more helpful error.