Modify the handling of exit to throw the correct code

[Thirumalai-Shaktivel]

I recently came across something like this:

def test():
	quit(10)

test()

$ lpython a.py
err = 2560
STOP
$ echo $?
1

So, I thought to replace this with the following change:

                if (err != 0) {
                    if (0 < err && err < 256) {
                        return err;
                    } else if (err % 256 == 0) {
                    	return err / 256;
                    } else {
                        return 1;
                    }
                }

@certik

Originally posted by @Thirumalai-Shaktivel in #1302 (comment)