XStream silently ignores unknown elements

Config.java:
public List preprocessors;
Invalid config file:
<preprocessor class="pp1"/>
<preprocessors>
<preprocessor class="pp1"/>
</preprocessors>

When reading this xml-file, XStream silently ignores the first preprocessor element.
Instead it should thrown an exception.

By looking at com.thoughtworks.xstream.converters.reflection.AbstractReflectionConverter#doUnmarshal
it's found that it correctly deserializes the <preprocessor> element because the type is determined
by the class-attribute. As their is no field "preprocessor" defined in Config.java, it calls writeValueToImplicitCollection()
but this does nothing.

So the recommended work-around is throwing an Exception in writeValueToImplicitCollection() if the related
field could not be found.