> And why some very, very special languages have an effectively-global variable called "errno" that you have to check after the call manually, and worry about whether maybe it was populated from some previous error.

As you can read at https://en.wikipedia.org/wiki/Errno.h errno is barely used by the C standard (though defined there). It is rather POSIX that uses errno very encompassingly. For example the WinAPI functions use a much more sensible way to report errors (and don't make use of errno).