[TASK] Clarify exception message for addPlugin()

Add verbosity to the exception message to make it
more clear what is going wrong.

Releases: 6.2, 6.3
Resolves: #60225
Change-Id: I7bb4a16ff94d05b8b97633383d42c61aa600b1cf
Reviewed-on: https://review.typo3.org/31562
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel