Restructure error and success messages for the editor.

closes #1368
- merged the Maps for Error/Notification Messages into one JSON-structure
- this structure is more translation-friendly and influenced by I18n-standards.

Merged the Error and Message Map into one structure

The new JSON-structure is more translation friendly and similiar to i18n.
Merge pull request #1410 from schmanu/iss1368

Added new notification message on updating a post.