Add exception when the session key cannot be retrieved from the db

[modulitos]

(Migrated from the doc: https://docs.google.com/document/d/1qenf2HZAv6OnpyU0tUbh0ZfeJG_Rqb_MfkcNqd_YgTk/edit)

this error when (I think) there's a non-admin user:

/Users/sethvincent/workspace/flatsheet/flatsheet/routes/sheets.js:26
      if (account.admin) {
                 ^
TypeError: Cannot read property 'admin' of undefined
    at /Users/sethvincent/workspace/flatsheet/flatsheet/routes/sheets.js:26:18
    at /Users/sethvincent/workspace/flatsheet/flatsheet/lib/permissions.js:12:21
    at /Users/sethvincent/workspace/flatsheet/flatsheet/lib/index.js:117:30
    at /Users/sethvincent/workspace/flatsheet/flatsheet/node_modules/cookie-auth/index.js:46:21
    at dispatchError (/Users/sethvincent/workspace/flatsheet/flatsheet/node_modules/subleveldown/node_modules/levelup/lib/util.js:77:7)
    at /Users/sethvincent/workspace/flatsheet/flatsheet/node_modules/subleveldown/node_modules/levelup/lib/levelup.js:222:14

[modulitos]

I was unable to reproduce the last error on that Google Doc. It seems to occur when permissions.authorizeSession(req, res, function (err, account, session) { executes the callback with a falsey err and account is undefined. This means that self.accountdown.get(session.data.username, function (accountError, account) { cannot find the session, but does not return an error. Thus, the sessions would be in some kind of an unusual state where the session key does not have a value. I can add an extra check that covers this issue.