> Undefined behavior isn't bad, it's a unavoidable consequence of all languages, programming or otherwise.

This is incorrect.

"Undefined behavior" is a specific technical term for a scenario in a program that the compiler is permitted by the specification to assume will not happen, for the purposes of optimization. For instance, this code:

    int silly(int a) {
        if (a + 5 > a) return 0;
        return 1;
   }

can be optimized to just "return 0", because, as a human would read it, obviously a + 5 > a. So the spec says that signed integer overflow cannot occur to allow the compiler to optimize this as a human would want the compiler to optimize this. (Whether the spec actually matches human expectations is a good question, but in general it's right, and forbidding all undefined behavior in C and C++ would cause you to miss out on tons of optimizations that you obviously want.)

"Undefined behavior" does not mean providing invalid input to a function and getting an exception, or a crash, or a particular error result, if the result is well-defined. For instance, this is defined behavior:

    >>> import math
    >>> math.sqrt(-1)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    ValueError: math domain error

because Python defines the behavior that math.sqrt(-1) raises a ValueError and I can reliably catch that exception. If it were undefined behavior, then I wouldn't have any guarantee of being able to catch the ValueError: the Python interpreter might just choose to return 27 if it's more convenient.