Add exception serialization tests

These are pretty comprehensive tests for the exception serialization
code. The code itself code do with a bunch of refactoring, but that will
come later.

The main TODO item is that we need a way for users of the API to
register the list of modules that it wants to allow exceptions to be
deserialized from. There is no need for this to be an end-user
configuration option, but we do need each service to be able to specify
its own list.

Change-Id: Ib2fc3f1ca5da708b41bdc00e8ac6ce0c92399ce8
Merge "Add exception serialization tests"