Handle cases where `message_args` is a list.

This can be reproduced by redefining a variable in a list comprehension. This
produces the error message `list comprehension redefines %r from line %r` with
message args `[xx, 120]` (a list).
Merge pull request #1 from lakshmivyas/bug/message-formatting

Handle cases where `message_args` is a list.