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
```
@@ -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)
