Need a policy about missing or incompatible units

In current SIS version, unmarshalling of DefaultPrimeMeridian or DefaultEllipsoid object with missing or wrong units will throw a NullPointerException or ClassCastException at unmarshalling time. We need to either provide some mechanism for emitting a warning and set a default value (is a default value really desired? it may be dangerous), or wrap the above exception into something more informative.