cmake_policy: Failing minimum cmake version constraint doesn't stop further processing

While comparing the behavior of cmake_minimum_required(VERSION min...max) and cmake_policy(VERSION min...max), it was discovered that the former will halt further processing if the currently used CMake version is less than min, but the latter does not. Both issue an error message and ultimately result in the configure stage failing, but cmake_policy() shouldn't allow further processing if the minimum CMake version constraint is not met.

I think the problem is that the return value of SetPolicyVersion() is discarded instead of being returned to the caller here.