More exception policies

The ones that have been suggested:

1. a policy with a uniform, guaranteed, compiler-friendly UB, like `__builtin_unreachable`.
2. a policy that calls `std::terminate()` (and enables the inspection of the core or launches debugger.

No files, tests, or entry points are named. Review the suggested policies and the surrounding issue discussion first, then determine the intended policy interface and semantics; done means the requested exception-policy behavior is clearly specified and implemented with coverage for both proposed cases.

Written by the indexing model from the issue text.