Add backend form error messages

Currently global form errors (extra fields, csrf, etc) are not displayed in backend.
This commit adds one to discuss if this is the desired approach. (will do all if OK).
Add global form errors to backend forms
Adding global form errors to some Frontend forms
Merge pull request #346 from Richtermeister/backend-form-errors

Add backend form error messages
New backend theme with Bootstrap 3
Update translations
Update translations, start reworking the product page
Some fixes and adjustments
Add initial breadcrumbs
Additional polishing, some hardcoded styles removal.
More breadcrumbs
Initial translations
Add missing breadcrumbs
Fix missing translations
Fix some broken scenarios
Fix address and user searching features
Cleanup css a bit, hide button text on small devices, adjust some margin/padding on small devices
Add missing icons and breadcrumbs. Fix rendering of shipping method form
Polished filter forms rendering
Fix breadcrumbs html, make sidebar full page size, fixed rendering of variant creation/editing