Causing an error when session-data-defaults is malformed.

[nataliecarey]

This resolves the reported issue #2416

To reproduce this, edit the file app/data/session-data-defaults.js in a way that breaks it. My test file contents was to add one line containing a syntax error to the default file:

module.exports = {
  'broken'
  // Insert values here

}

If that file is then deleted then the default value of {} is used.