Encouraging `unknown` type introduction before narrowing

[thierrymarianne]

Hi!

In the spirit of providing alternate solutions
for exercice 13 (entitled catch-blocks),
does it make sense to encourage possible introduction of

• unknown type
• guard clause for case when e variable inheriting
  from Error class so that it is handled
  as expected (with expectation provided by test assertion),
• guard clause for case when e variable
  does not inherit from Error class?