readline: fix freeze if `keypress` event throws

`emitKeys` is a generator which emits `keypress` events in an infinite
loop. But if `keypress` event handler throws, the error stops the loop,
leaving generator in a broken state. So this patch restarts the generator
when an error occures.

PR-URL: https://github.com/nodejs/io.js/pull/2107


Reviewed-By: Christopher Monsanto <chris@monsan.to>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>