[BUGFIX] Fix wording of exception in DataMapper

The message of the exception introduced with #89857 needs some
improvements.

Resolves: #90996
Releases: master, 9.5
Change-Id: If0bc77ac8bcdab44a6468dc7bd1eb3543673e7a8
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64116


Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Christian Eßl <indy.essl@gmail.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Josef Glatz <josefglatz@gmail.com>
Reviewed-by: Christian Eßl <indy.essl@gmail.com>