Document Map.fetch!/2 exception in the doc test

Also mirrors `Map.fetch/2` and thought it was nice to have there
for easy & quick overview purposes.

Author: PragTob

lib/elixir/lib/map.ex

@@ -316,6 +316,9 @@ defmodule Map do
 
       iex> Map.fetch!(%{a: 1}, :a)
       1
+      iex> Map.fetch!(%{a: 1}, :b)
+      ** (KeyError) key :b not found in:
+      ...
 
   """
   @spec fetch!(map, key) :: value