Resume throwing in destrujctors when no uncaught exceptions are in flight

[eyalroz]

We have recently made sure none of our destructors throws (unless a preprocessor definition is set): #728 . However - that is actually a bit too strict of a policy, for avoiding termination. Rather we can use std::uncaught_exception() (or uncaught_exceptions() in C++17 and later) to decide whether or not we can safely throw. Let's do that.