Throwable can't have its fields serialized

[niloc132]

In GWT 2, only detailMessage was serialized, but since old serialization relied on gwt generators, we only saw the source that was provided to GWT, instead of the types in the jre.

A CustomFieldSerializer should be sufficient to read the detailMessage value, but each subclass will need to create the exception with that message with its constructor if possible. Alternatively, providing wiring for the violator pattern in this specific case might help, or leveraging explicit final support to try to pass this to constructors.