fun1-intro: move fails after successes

[DavidLeoni]

Currently, first we show fails like and only afterwards the proper 'successful' way, but I feel explanation gets too lengthy.

Instead, we can turn the fails into exercises (or maybe questions?)

https://en.softpython.org/functions/fun1-intro-sol.html#Changing-the-world:-fail-/-1

https://en.softpython.org/functions/fun1-intro-sol.html#Doubling:-fail-/-1