Add checks for packages and contentPath to startup

Refs #3864
- Make sure that require() is able to resolve all dependencies
  listed in package.json.  If packages are missing halt the
  bootstrap process and display an error and help message for user.
- Check that contentPath and its subdirectories exist with the correct
  permissions.
- Check sqlite3 database file is set for read/write access.
Merge pull request #3913 from jaswilli/require-errors

Add checks for packages and contentPath to startup