[BUGFIX] Validation with invalid validator caused unrelated errors

This change checks the resolved validator class name for presence
of the correct interface before accepting it as a validator.

Fixes: #11374
Releases: 6.0, 6.1

Change-Id: I3956ba85b01e6256ffdce9fe50be98216412b15c
Reviewed-on: https://review.typo3.org/7149
Reviewed-by: Andreas Wolf
Tested-by: Andreas Wolf