Better message for memory leaks errors

bob/io/video/test.py

@@ -123,7 +123,7 @@ def test_memory_leak():
           first = psutil.virtual_memory().used
     last = psutil.virtual_memory().used
 
-  assert (last - first) / first < 0.1, "Looks like we have a memory leak!"
+  assert (last - first) / first < 0.1, f"Looks like we have a memory leak! first:{first} last:{last}"
 
 
 def test_open_file_leak():