This section is a bit of an indentation mish-mash. PEP 8 contains an example that suggest something more like:
```
    if (cs is not None and
        not isinstance(cs, (iris.coord_systems.GeogCS,
                            iris.coord_systems.RotatedGeogCS))):
        msg = "Latlon coords cannot be found with {0}.".format(type(cs))
        raise ValueError(msg)
```
Perhaps the pair of valid coordinate system classes could be stuck in a temporary variable as well.