If the app is not installed, show error.

Error page will bring user to the Chrome webstore link.
Merge remote-tracking branch 'origin/dev' into lucyhe-app_missing_popup
Set "waitingForAppInstall" to false after install.
Remove unused imports.
Add Chrome webstore link.
Fix ChromeConnector test.
Open extension on install.
Merge pull request #685 from uProxy/lucyhe-app_missing_popup

Use extension popup to direct user to install app.