teuthology/suite/matrix.py: Add item to assert msg

Give folks at least some hint about where things might
be broken:

```
AssertionError: Sum requires non-empty _submats: 0-distro
```

Signed-off-by: Sebastian Wagner <sewagner@redhat.com>

diff --git a/teuthology/suite/matrix.py b/teuthology/suite/matrix.py
index 8938fe7d868353b794f3fa7826b5e5d52ee2deda..055c4485232cec67f18c7812fe3e094f27d99354 100644 (file)
--- a/teuthology/suite/matrix.py
+++ b/teuthology/suite/matrix.py
@@ -248,7 +248,7 @@ class Sum(Matrix):
     """
     def __init__(self, item, _submats):
         assert len(_submats) > 0, \
-            "Sum requires non-empty _submats"
+            f"Sum requires non-empty _submats: {item}"
         self.item = item
 
         self._pseudo_size = lcml((i.size() for i in _submats)) * len(_submats)