Support for 'nonull': 'error'

[tkambler]

This PR is similar to the one that's been sitting here for nearly two(!) years. It's a simple six line change that allows users to specify that the task should throw an error via grunt.fail.fatal() when a missing file is encountered. It is entirely backwards-compatible. You have three options:

{ 'nonull': false } // Don't warn about missing files
{ 'nonull': true } // Warn about missing files
{ 'nonull': 'error' } // Throw missing files

[tkambler]

Test / documentation. Can I get a merge?

[tkambler]

If anyone else wants this functionality, FYI - grunt-concat-verify.