Implement checks for raising messages with exceptions (#1113)

* Implement checks for messages being raised with exceptions
(Fixes #1080)

* Add self.assertRaisesWithMessage method to relevant exercise tests
    - Uses self.assertRaisesRegex
    - Checks only for the presence of a message, not content
* Add meaningful messages to failing examples
* octal: Switch to using a context manager for exception tests

* Add note regarding error messages to the insert

* simple-linked-list: Move hints.md to correct location

* simple-cipher: Remove extra whitespace from lines

* collatz-conjecture: Update hints.md

* Regenerate README to include exceptions section

exercises/simple-linked-list/example.py

@@ -42,7 +42,7 @@ class LinkedList(object):
 
     def head(self):
         if self._head is None:
-            raise EmptyListException()
+            raise EmptyListException("The list is empty")
         return self._head
 
     def push(self, value):
@@ -53,7 +53,7 @@ class LinkedList(object):
 
     def pop(self):
         if self._head is None:
-            raise EmptyListException()
+            raise EmptyListException("The list is empty")
         self._len -= 1
         ret = self._head.value()
         self._head = self._head.next()