Minor: chain exceptions in renew_eyes

anonstream/user.py

@@ -285,8 +285,8 @@ def create_eyes(timestamp, user, headers):
 def renew_eyes(timestamp, user, eyes_id, just_read_new_segment=False):
     try:
         eyes = user['eyes']['current'][eyes_id]
-    except KeyError:
-        raise DeletedEyes
+    except KeyError as e:
+        raise DeletedEyes from e
 
     # Enforce expire_after (if the background task hasn't already)
     renewed_ago = timestamp - eyes['renewed']