Fixes #12 Missing variables on string formatting

In the "No valid path" strings it prints %s instead of node names

command.py

@@ -942,7 +942,7 @@ class MetricShell(Cmd):
       length, paths = self.simulation.path(a,b)
 
       if not length and not paths:
-         print("No valid path from %s to %s in model")
+         print("No valid path from %s to %s in model" % (a,b))
          return
 
       print("Path from %s to %s:"\
@@ -1129,7 +1129,7 @@ class MetricShell(Cmd):
       length, paths = self.model.path(a,b)
 
       if not length and not paths:
-         print("No valid path from %s to %s in model")
+         print("No valid path from %s to %s in model" % (a,b))
          return
 
       print("Path from %s to %s:"\