Merge branch 'protocol-state-exception' into 'master'

ProtocolStateException for client protocols

Methods that implement local actions in a client protocol (for example, accepting an invitation) can throw this exception to indicate that the action wasn't taken because the action isn't applicable to the current state. This can happen if the protocol state machine is updated by an incoming message and the user takes an action before the UI has been updated.

See merge request !368