post_process.redirect: Log exceptions as warnings

post_process.redirect: Log exceptions as warnings

When we hit an exception trying to unwind any redirects, just print
the exception and leave the link with its original value.

This commit also alphabetizes the standard library imports, and splits
off the local import following PEP 8's suggested groupings [1].

[1]: http://legacy.python.org/dev/peps/pep-0008/#imports

Signed-off-by: W. Trevor King <wking@tremily.us>