Bring back --message option and MaintenanceModeException for Maintenance Mode

[taka-oyama]

The new implementation since v8 removed the ability to set a message and instead allowed for templates.
This works great for HTML but it either return HTML in a JSON request or it gives the default 'Service Unavailable' error message.

Is it possible to get MaintenanceModeException and --message option back in so it can be handled better in Exception Handler?