[BUGFIX] Missing information in debug output

Restore display of message header and content in the new
FE debug output.

Resolves: #58686
Releases: 1.6
Change-Id: Idb4670177b80a870ef1c4d691b9d73853f61cf05
Reviewed-on: https://review.typo3.org/29980
Reviewed-by: Francois Suter
Tested-by: Francois Suter