Fix misnamed dictionary key.

The response dictionary in _delete_segment attach's any error
it encounters to the dict key 'exception', all other response
dict's use 'error' to store the exception.

Changed to make it consistent and added tests

Also added a third branch in st_delete as messages
created in _delete_container where being silently dropped.

Change-Id: Ifbc3b1fae78910fbc6acf4a86cfb0f60bb1aa336
Merge "Fix misnamed dictionary key."