Make AnsibleError a plain ol' exception

Python's Exception constructor already takes a `message` as a parameter,
which you can then get at by doing str(e) (e.message was deprecated).

The reason I bothered to make this change was because I was debugging
with pdb and I noticed that AnsibleErrors don't give useful information
in pdb (probably because they don't have a __repr__ method that prints
the `msg` attribute).

    (Pdb) c
    > /Users/marca/dev/git-repos/ansible/lib/ansible/runner/__init__.py(599)_executor()
    -> msg = str(ae)
    (Pdb) ae
    AnsibleError()

lib/ansible/errors.py

@@ -17,12 +17,7 @@
 
 class AnsibleError(Exception):
     ''' The base Ansible exception from which all others should subclass '''
-
-    def __init__(self, msg):
-        self.msg = msg
-
-    def __str__(self):
-        return self.msg
+    pass
 
 class AnsibleFileNotFound(AnsibleError):
     pass