Fixed promise error handling

catch was being called on the wrong object, so some errors were not being caught
Renamed m to mod

so it doesn't conflict with var names within promise handlers
More error handling cleanup

calling next() on errors within promises.
got rid of inner errors. responding with the actual errors thrown instead
Added 'load' query param to load relations

calls the Bookshelf.js load() method on a model, to load related models.