read and tail take different kinds of callbacks

[pvh]

Not sure why tail() doesn't have an err callback but it was a bit surprising.

[hackergrrl]

I agree. tail and onInsert could be emitted events instead:

view.events.on('insert', (msg) => {})

view.events.on('tail', (msg) => {})

view.events.on('error', (err) => {})