[TASK] Throw InvalidArgumentException for undefined log level

PSR-3 expects Logger->log() to throw Psr\Log\InvalidArgumentException
when an invalid log level was given.

Resolves: #48876
Releases: 6.2
Change-Id: I89637e72acdce1b1a9e56b838b632d6b486dcae9
Reviewed-on: https://review.typo3.org/21847
Reviewed-by: Markus Klein
Tested-by: Markus Klein