Small improvements to docularserver

Hey Stephen,

I made a couple of small improvements to docular server based on my development experience as listed below, see if you can merge them:

1.  use sendFile instead of sendfile to avoid deprecation error
2. Send site.json and structure.json via sendFile and not express static to avoid issues with different OS's (windows).
3. return 404 on files that are not found inside the resource folders to ease debugging
4. fallback to docular grunt config to try and figure out the target folder
5. throw an error if not target folder found that explains what needs to be fixed