message: Improve error message when a function is already defined with the same name and different hash string.
diff: @@ -2952,7 +2952,11 @@ class Graph(object):
  if previous._hash_str == function._hash_str:
  return
  else:
  - raise ValueError("Another function is already defined with that name")
  + raise ValueError("Cannot add function (%s, hash %s) to graph (%s). "
  + "Another function (%s, hash %s) is already defined "
  + "with that name (%s)" % (
  + function, function._hash_str, self,
  + previous, previous._hash_str, name))
  # pylint: enable=protected-access
  self._functions[name] = function
  