It's goofy, but one of the reasons I'll pick C for a quick program is that both Windows and Linux will give better error messages for standard functions that fail and set errno than for equivalent C++ code that throws an exception.

This isn't a question about the language, it's definitely a quality-of-implementation issue for standard exceptions to have what() give a useful message.

Just to clarify perror will generally give a better error message in:

    FILE* fh = fopen("foo.txt", "w+");
    if (!fh) {
        perror("unable to open foo.txt");
        return 1;
    }

Than what() will in:

    std::fstream fs;
    fs.exceptions(std::ifstream::failbit | std::ifstream::badbit);
    try {
        fs.open("too.txt");
    }
    catch (const std::exception& ex)
    {
        std::cerr << "unable to open foo.txt: "
           << ex.what() << '\n';
    }

(Ignoring other issues such as whether exceptions are better than return codes, etc.)

A lot of people seem to use stdio even in C++.

Having comprehensible error messages is related to being a simple language, IMO. C++ compile-time errors are notorious for being unreadable. I'm not familiar with the output of what(), but I'd wager that the problem is a consequence of all the abstraction.