Update exceptions.md (#1431)
Update exceptions.md (#1431)

Documented error handling from ``app.error_handler.add``
Documented custom error handling by subclassing.
channelcat/sanic,channelcat/sanic,lixxu/sanic,lixxu/sanic,lixxu/sanic,ashleysommer/sanic,ashleysommer/sanic,channelcat/sanic,yunstanford/sanic,channelcat/sanic,yunstanford/sanic,ashleysommer/sanic,yunstanford/sanic,yunstanford/sanic,lixxu/sanic