[BUGFIX] Handle exceptions in Logging API

Do not run exceptions through json_encode() but convert them to strings
first.

Resolves: #65577
Releases: master, 6.2
Change-Id: I46e743480b32010bff39784a3572a5fe96cf0859
Reviewed-on: http://review.typo3.org/37602


Reviewed-by: Steffen Müller <typo3@t3node.com>
Tested-by: Steffen Müller <typo3@t3node.com>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>