Rework the put_update_batch() interface

This commit makes the interface of put_update_batch() quit similar to
post_new_batch().

setra_client/client.py

@@ -386,13 +386,18 @@ class SetraClient(object):
                             data=batchdata.json(exclude_unset=True),
                             headers=headers,
                             return_response=True)
+
+        try:
+            content = response.json()
+        except requests.exceptions.JSONDecodeError:
+            content = response.content
+
         if response.status_code == 204:
-            return response.content, 'No Content'
+            return 'Accepted', {}
         elif response.status_code == 409:
-            return response.content, 'batch_found_but_cannot_be_updated'
+            return 'Conflict', content
         else:
-            response.raise_for_status()
-        return response
+            return 'Unknown', {'code': response.status_code, 'content': content}
 
     def get_batch_complete(self, batch_id: str):
         """