Improve app-management

- Better error messages

- Translate untranslated strings

Basically picks non-app-dependency related stuff from #4017
Log exception at the catching code
Merge branch 'master' into improve_app-management
Throw exceptions instead of only logging in \OC_Installer::installApp()
Merge branch 'master' into improve_app-management
Don't use an alert for displaying app-mgmt-errors

Rather display a dominant div and mark the problematic app in the applist.

Fix #305
Use appitem instead of always recreating a jquery object

Also fix some wrong data storages
Move .errormsg from settings-css to .warning in core. Reusable!
Merge branch 'master' into improve_app-management

Conflicts:
	settings/js/apps.js
Merge pull request #4331 from owncloud/improve_app-management

Improve app-management