Warn if GetNestedType is not checked for null or offer refactoring to throw on error

[jnm2]

Should GetNestedType be treated as a special case of GetType so that the existing #14 check is applied?

That would be cool because I'd totally prefer a refactoring from type.GetNestedType to type.Assembly.GetType("RepeatedTypeName+NestedTypeName, throwOnError: true) over avoiding NRE manually.