Attempt to reauth on bad auth tokens

Apparently CouchDB recently stopped simply rejecting these, and instead
returns a 403 from the validate_doc_update function, rather than an
immediate 401.

While this is undoubtedly a great security feature (since they don't leak
as much info about the reason for the write rejection) it's a bit annoying
when the token cookie expires sometimes.

Hopefully this makes the reports go away.
v0.2.14