reformulate message of previous commit

kwant/builder.py

@@ -1071,7 +1071,7 @@ class Builder(object):
         the leads are numbered in the order in which they are attached.
         """
         if self.symmetry.num_directions:
-            raise ValueError("Can only attach leads to finite systems")
+            raise ValueError("Leads can only be attached to finite systems.")
 
         if add_cells < 0 or int(add_cells) != add_cells:
             raise ValueError('add_cells must be an integer >= 0.')