Exit with an error if a command cannot be sent from the CLI

[AlexGustafsson]

When a command is run via the CLI, the application will try to connect to the message broker service. If this connection fails, an error will be printed, but the application will not exit - potentially causing undefined behavior.

These errors rarely occur in reality, however.

connection.interruptionHandler = {
  print("Disconnected from broker (interrupted)", to:&stderr)
  // Exit in a sensible way
};

connection.invalidationHandler = {
  print("Disconnected from broker (invalidated)", to:&stderr)
  // Exit in a sensible way
};

[AlexGustafsson]

Implemented.