Minor fixes. Raise exception when method is not implemented

Change-Id: I0080263c36959f24311c474d7241108f8c24bd48
Signed-off-by: tierno <alfonso.tiernosepulveda@telefonica.com>
diff --git a/osm_common/msgbase.py b/osm_common/msgbase.py
index a9d44d7..98f97bf 100644
--- a/osm_common/msgbase.py
+++ b/osm_common/msgbase.py
@@ -35,13 +35,13 @@
         pass
 
     def write(self, topic, key, msg):
-        pass
+        raise MsgException("Method 'write' not implemented")
 
     def read(self, topic):
-        pass
+        raise MsgException("Method 'read' not implemented")
 
     async def aiowrite(self, topic, key, msg, loop):
-        pass
+        raise MsgException("Method 'aiowrite' not implemented")
 
     async def aioread(self, topic, loop):
-        pass
+        raise MsgException("Method 'aioread' not implemented")