Exception handling

[bedeho]

Background

The following exception are currently thrown from inside the machine

• CannotInviteNonSeller
• InvitedToJoinContractByNonBuyer
• StateIncompatibleEvent

Problem

We are not handling these in a principled way in protocol_session, and its not clear that they are all appropriate, nor what impact they may have on boost_statechart itself. See

http://www.boost.org/doc/libs/1_49_0/libs/statechart/doc/tutorial.html#ExceptionHandling