Convert trigger_error in VariationCache into a LogicException

Problem/Motivation

We introduced extra hardening in the VariationCache but for BC reasons could not throw an exception. We ended up going for a trigger_error with level E_USER_WARNING instead, but writing the tests in a way already that they expect a LogicException. This was achieved by having the tests user a custom error handler. See #3452181: VariationCache needs to be more defensive about cache context manipulation to avoid broken redirects

The goal is to remove this trigger_error in favor of an actual LogicException and to remove the small workaround from the tests.

Steps to reproduce

N/A

Proposed resolution

1. Convert trigger_error in VariationCache into LogicException
2. Remove custom error handler and try-catch in VariationCacheTest

Remaining tasks

Wait for D12 branch to become available

User interface changes

N/A

Introduced terminology

N/A

API changes

N/A

Data model changes

N/A

Release notes snippet

TBD