Do a longstanding TODO to remove kustomizationerror.
It wasn't used much, and it wasn't used consistently,
because it's complicated to decided when it's worth
proceeding to accumulate errors when one already knows
that one has a fatal error in the kustomization. Its
use was blocking refactoring for simplicity and making
tests harder to write.
Removing it lets us reinstate the cyclomatic complexity
check in KustTarget.
Also added more info to the affected error messages.
