Make errors example into a function

[SJaffa]

https://github.com/UoMResearchIT/programming_with_python/blob/19edfb758c78c9614ae7a2406dfa581a473f47ac/_episodes/05-defensive_programming.md?plain=1#L29

We could make this into a function so it is easier to add to it rather than copy-pasting, and clearer that the person giving the input value might be a different person.

[Scottan]

I thought the same when giving the course on Tuesday, would have let me focus on the important parts of the lesson more.

I also find the wording of the test a bit weird. I would recommend:

> def test_number(val):
>     if val == 0:
>         print('Value ', val, 'is zero')
>     elif val > -10 and val < 10:
>         print('Value: ', val, 'is a single digit.')
>     else:
>         print('Value: ', val, 'is a multi-digit number.')

Honestly, the test itself is fairly unimportant, although I think making it clear that negative values are ok is a good idea because we had a student ask if this was the issue. Checking for negative, 0 or positive would also be fine, and probably simpler so we keep it focussed on what's important for the lesson:

> def check_sign(val):
>     if val == 0:
>         print('Value ', val, 'is zero')
>     elif val > 0:
>         print('Value: ', val, 'is positive')
>     else:
>         print('Value: ', val, 'is negative')

[Scottan]

p.s. adding suggestion from @AnnieZQH to add a box in this section on use of try / except referring to:

https://docs.python.org/3.6/glossary.html#term-eafp