Improve watson dialogue tree

[ianhansborough]

• transitioning between trees gracefully
• add generic greetings, goodbyes, etc.
• watson-side error handling

[jacobpa]

I saw that someone had created a #cancel intent. I wired it so that for all of our dialog trees, if this intent is recognized when prompting for details, the dialog tree will exit. This is done in the slot configuration for each dialog action, under "Manage handlers"

Context variables persist, but should not be an issue as relevant context variables are always cleared before entering a dialog tree. It should not behave any differently than if the conversation was completed successfully, though watch for issues.