Exception policy for handling request, notification and message.

[veblush]

Current state of exception handling is:

• When handling a request, forward exception to requester and go on.
• When handling a notification and a message, follow akka exception policy (ask a parent to what to do)

The reason that request handling have a different exception policy is that it follows the RPC model and exception need to be propagated to caller. (not parent) But this inconsistency is not good to keep and sometimes a critical exception in handling a request requires parent to do something. It seems reasonable to change policy like following.

• When handlig a [request | notification | message], follow akka exception policy.
• There might be an option to make some kinds of exception propagated to a caller not a parent.
• Whenever an exception is raised in handling a request, a caller should get exception response to prevent a request timeout.