Fix typo for midplane APIs. (#196)

In midplane API, default method should be raise NotImplementedError not return NotImplementedError

Author: caizhenghui-juniper

sonic_platform_base/module_base.py

@@ -451,7 +451,7 @@ def get_midplane_ip(self):
             A string, the IP-address of the module reachable over the midplane
 
         """
-        return NotImplementedError
+        raise NotImplementedError
 
     def is_midplane_reachable(self):
         """
@@ -461,4 +461,4 @@ def is_midplane_reachable(self):
         Returns:
             A bool value, should return True if module is reachable via midplane
         """
-        return NotImplementedError
+        raise NotImplementedError