Throw proper error messages

Formerly this error was thrown when the path doesn't exist:

path.js:360
        throw new TypeError('Arguments to path.join must be strings');
              ^
TypeError: Arguments to path.join must be strings
    at path.js:360:15
    at Array.filter (native)
    at Object.exports.join (path.js:358:36)
    at FileSessionStore.set (/home/mop/public_html/rrr/node_modules/connect-session-file/lib/connect-session-file.js:151:25)
    at Session.save (/home/mop/public_html/rrr/node_modules/express/node_modules/connect/lib/middleware/session/session.js:63:25)

That's not really helpful ;)