Well done spotting this obscure bug!!

It took me a while to figure out the impact though. It might make it easier to understand if we move the code and comment to [just before the try-except block](https://github.com/esc24/iris/blob/6001790150024b7de927e14550414e11c34f444d/lib/iris/fileformats/rules.py#L735). And perhaps change the comment to something like, "'grids' is a list of tuples of coordinates, so convert 'target_coords' to be consistent."?