Provide useful error message when styles/app.ext is not found

To avoid future developer confusion as witnessed on issue #1641
Update ember-cli-qunit.
Merge pull request #1877 from rwjblue/update-ember-cli-qunit

Update ember-cli-qunit.
Update CHANGELOG.md.

Updates to changes as of e4e13cc.
Move `tmp-sync` to a dev dependency.

It is approximate 2MB, and is not used in non-test code.
Merge pull request #1882 from rwjblue/move-tmp-sync-to-dev-dep

Move `tmp-sync` to a dev dependency.
Do not require build related packages for every boot.

Requiring these at the top of the file means that for every start of the
`ember` binary these packages are loaded up.

* Doing the requires lazily speeds things up.
* Requiring `broccoli-es6-concatenator` at boot makes `ember-cli` >
  0.0.40 impossible to use for building Ember (or any tool that uses
  an alternate ES6 concatenator) due to a bug with `es6ify`.
Merge pull request #1883 from rwjblue/speedup-boot

Do not require build related packages for every boot.
Do not throw an error if no vendor/ folder exists.
Add `npm install` to RELEASE.md steps.
Merge pull request #1884 from rwjblue/do-not-error-without-vendor-folder

Do not throw an error if no vendor/ folder exists.
Bump version to 0.0.43.
[Bugfix] this.pickFiles did not exist
Merge pull request #1892 from stefanpenner/pick-files-fix

[Bugfix] this.pickFiles did not exist on addon
use silenterror
Merge pull request #1894 from stefanpenner/enhance-pr/1866

Enhance pr/1866
Updated dependency broccoli-es3-safe-recast
Updated dependency broccoli-merge-trees
Updated dependency fs-extra
Updated dependency proxy-middleware