Fail more gracefully if missing auth.digestFile or app.mongoUrl settings
Disable session cookie path

For some reason, this causes sessions to not be initialized properly.

Also remove an unused res.locals statement.
Use res.locals.namespace instead of config.app.namespace

config.app.namespace may not be defined, but res.locals.namespace will.
Fix previous commit

res.locals.namespace was being used outside of a route, where it's not
available.