Automatically inform the user on internal node error

[nemanja-boric-sociomantic]

As discussed in #213 and #214, it is proposed to make sure node request handlers catch all exceptions and notify the client that there was an error. (i.e. request handlers should never throw.)

This can be implemented on the swarm level in the following way:

• On the node side, catch the exceptions and send the message to the client with the different MessageType - so that the client doesn't need to be always registered for receiving error messages. However, as Handling of I/O exceptions in node request handlers #216 states, we shouldn't catch IOErrorin the RequestOnConn's fiber handler method, since the client will anyway be notified by the connection dropping.

• On the client side, it's an open question how this message will be handled, since the notifiers are handled by the request, not by the connection handler in swarm. One possibility would be just to resume the request fiber with an exception, but that requires try/catch behaviour on the client side.

[gavin-norman-sociomantic]

Moved to future. It sounds like this would be difficult to implement, and might require a new protocol version.