Update exceptions

Add NotImplemented exception and fix flake8 issues (spacing)

Change-Id: I675ff7156084c41f1dc3b5ed4823f237e0835ad1
Signed-off-by: Adam Israel <adam.israel@canonical.com>
diff --git a/n2vc/exceptions.py b/n2vc/exceptions.py
index fd4a3ce..a63d6f9 100644
--- a/n2vc/exceptions.py
+++ b/n2vc/exceptions.py
@@ -39,5 +39,11 @@
 class AuthenticationFailed(Exception):
     """The authentication for the specified user failed."""
 
+
 class InvalidCACertificate(Exception):
-    """The CA Certificate is not valid."""
\ No newline at end of file
+    """The CA Certificate is not valid."""
+
+
+class NotImplemented(Exception):
+    """The method is not implemented."""
+