Fix following training session from the 20th of May 2021

Issues #21 (closed) and #22 (closed) are tackled in this MR.

5-UsefulConceptsAndSTL/1-ErrorHandling.ipynb

     catch(int n)
     {
         if (n == 1)
-            std::cout << "Error: value is bigger than 9!" << std::endl;
+            std::cerr << "Error: value is bigger than 9!" << std::endl;
         if (n == -1)
-            std::cout << "Error: value is less than -9!" << std::endl;
+            std::cerr << "Error: value is less than -9!" << std::endl;
     }
     std::cout << "End" << std::endl;
 }
     }
     catch(float n) // doesn't catch your `int` exception!
     {
-        std::cout << "Float case: " << n << " was provided and is not an integer" << std::endl;
+        std::cerr << "Float case: " << n << " was provided and is not an integer" << std::endl;
     }
     catch(...)
     {
-        std::cout << "Gluttony case... but no object to manipulate to extract more information!" << std::endl;
+        std::cerr << "Gluttony case... but no object to manipulate to extract more information!" << std::endl;
     }
 }
     }
     catch(int n)
     {
-        std::cout << "Int case: " << n << " not a single digit number" << std::endl;
+        std::cerr << "Int case: " << n << " not a single digit number" << std::endl;
     }
 
     std::cout << "After catch" << std::endl;
     }
     catch(int n)
     {
-        std::cout << "Int case: " << n << " not a single digit number" << std::endl;
+        std::cerr << "Int case: " << n << " not a single digit number" << std::endl;
         throw; // `throw n` would have been correct as well but is not necessary
     }
 
     }
     catch(const std::exception& e)
     {
-        std::cout << "Properly caught: " << e.what() << std::endl;
+        std::cerr << "Properly caught: " << e.what() << std::endl;
     }
 }
 
 with the latter being the default choice if no other fit your issue. Most of those classes provide a `std::string` argument in its constructor so that you may explain exactly what went wrong.
 
+The previous case could be rewritten as:
+%% Cell type:code id: tags:
+``` C++17
+#include <iostream>
+#include <exception>
+
+void FunctionThatExpectsSingleDigitNumber3(int n)
+{
+    if (n < -9)
+        throw std::out_of_range("Value is less than -9!");
+
+    if (n > 9)
+        throw std::out_of_range("Value is more than 9!");
+
+    std::cout << "Valid digit is " << n << std::endl;
+}
+%% Cell type:code id: tags:
+``` C++17
+FunctionThatExpectsSingleDigitNumber3(-10);
+%% Cell type:code id: tags:
+``` C++17
+FunctionThatExpectsSingleDigitNumber3(3);
+%% Cell type:code id: tags:
+``` C++17
+FunctionThatExpectsSingleDigitNumber3(15);
+%% Cell type:markdown id: tags:
 
 ### `noexcept`
 
 Exceptions are in fact very subtle to use; see for instance \cite{Sutter1999} and \cite{Sutter2002} that deal with them extensively (hence their title!).
 
 The explanation is quite subtle and explained in detail in item 8 of \cite{Meyers2005}; however just know you should never throw an exception there. If you need to deal with an error there, use something else (`std::abort` for instance).
 
-
-### The exception class I use
-
-I (Sébastien) provide in [appendix](/notebooks/7-Appendix/HomemadeException.ipynb) my own exception class (which of course derives from `std::exception`) which provides additionally:
-
-* A constructor with a string, to avoid defining a verbosy dedicated exception class for each case.
-* Better management of the string display, with an underlying `std::string` object.
-* Information about the location from where the exception was thrown.
-
-Vincent uses up the STL exceptions described in [previous section](#Good-practice:-be-wary-of-a-forest-of-exception-classes).
-
 ## Error codes
 
 A quick word about C-style error management which you might find in use in some libraries: **error codes**.