when a failure is not serializable, try to indicate which exception type was responsible

[davidsarah]

A common reason to get a foolscap.tokens.Violation is that some exception type transitively references an unserializable object. However the traceback doesn't provide any information about where to start looking, i.e. which exception type. In some cases, even without trying to fix the unserializability, just knowing the exception type would be a sufficient clue to the cause of the original error.