Add a new break on error

A user running di-b several times while developing an element may not
want to drop to a shell in all cases but may only want to do so if one
of their in target hooks failed.

This patch gives them the ability to do so, If break=after-error is set
then a user will be provided a in target shell taking over from where the
last failed command left off.

Change-Id: Ia2f7ac4c21b64b971f87f4ae9cb867981b13eb5e