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

To avoid future developer confusion as witnessed on issue #1641
[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
Updated dependency tiny-lr
Merge pull request #1898 from rwjblue/update-deps

Update deps (replace #1891).
cleanup http-proxy blueprint

* Proxy makes jshint unhappy “redefines global”
* add missing semi-colon
* require it on-demand, as to not negatively impact cli boot
* add missing path
Merge pull request #1896 from stefanpenner/cleanup-proxies

cleanup http-proxy blueprint
Update broccoli-caching-writer.
Merge pull request #1901 from rwjblue/update-caching-writer

Update broccoli-caching-writer.
Change bowerDirectory assignment order in case .bowerrc doesn't contain `directory`
Merge pull request #1902 from jayphelps/patch-3

Change bowerDirectory assignment order in case .bowerrc doesn't contain `directory`
Remove `window.MyAppNameENV`.
Update changelog.

Updated to 39c892e.
Bump version.