message: Slightly improve the error message when attempting to replace a keyword argument with something unsupported.
diff: @@ -90,7 +90,8 @@ class ReplaceTransformer(gast.NodeTransformer):
  # we could allow changing just node arg, so that we end up with bar=baz.
  raise ValueError(
  'a keyword argument may only be replaced by another keyword or a '
  - 'non-empty list of keywords. Found: %s' % repl)
  + 'non-empty list of keywords. Found: {} for keyword {}'.format(
  + repl, node.arg))
  def visit_FunctionDef(self, node):
  node = self.generic_visit(node)
  