better error handling for broken database.json files

I left out a comma in my database.json file the first time I tried to run an update. I got an error like:

  Cannot find module '/Path/to/Project/Path/to/project/database.json'

which made me think that the problem was with the location of my config file or with node-db-migrate itself. This pull request throws a syntax error if the file is found in the default location but cannot be read. Thanks.