Fix #739: tree: make expand return a promise
Improve tree expand promise tests to run fully, and add rejection test
tree: Remove call to _trackError around renderQuery in expand

The call is not necessary because the call to the query inside of
renderQuery is wrapped by _trackError.

The tree-expand-promise test now verifies the error event is being generated
and I added tests for grids that do not have an implementation of renderQuery.
tree-expand-promise unit test cleanup

* Fix typos and assertion phrasing
* Clean out unnecessary this.async usage
* Suppress dgrid-error logging by canceling event
CHANGES.md: Note tree expand method now returns promise