added radians test to cartopgraphy

This PR checks the coordinate units before attempting the conversion to radians. I've not added tests as I'm unsure.
If you put the string in parentheses you don't need the explicit line continuation (this is considered good style usually), for example:

```python
msg = ("Units of degrees or radians required, coordinate "
       "{!r} has units {!r}".format(coord.name(),
                                    coord.units.name))
```