IBM Storwize: Improve error message

Message "Unable to retype: Volume is replicated." can not really reflect
the reason why retype failed. We need to add additional information to
help user to better understand why retype failed.

Change-Id: If68bb1ba1e72df46fef35394c9ad207ffdeb271b

cinder/volume/drivers/ibm/storwize_svc/__init__.py

@@ -894,8 +894,9 @@ def retype_iogrp_property(volume, new, old):
 
             # If volume is replicated, can't copy
             if new_type_replication:
-                msg = (_('Unable to retype: Volume %s is replicated.'),
-                       volume['id'])
+                msg = (_('Unable to retype: Current action needs volume-copy,'
+                         ' it is not allowed when new type is replication.'
+                         ' Volume = %s'), volume['id'])
                 raise exception.VolumeDriverException(message=msg)
 
             retype_iogrp_property(volume,