Need a 'nonull' file option that also fails on missing files

Something like `nonullFail`:

```
bar: {
files: [
// if any files are missing, fail the build
{src: ['src/bb.js', 'src/bbb.js'], dest: 'dest/b/', nonullFail: true}
],
},
```

ref: https://github.com/gruntjs/grunt-contrib-concat/pull/70

Start by reviewing Grunt's file-option handling and the behavior described in the example configuration. Compare the requested nonullFail option with the referenced grunt-contrib-concat pull request. Done means an opt-in option causes the build to fail when any listed source file is missing, while existing behavior remains unchanged otherwise.

Written by the indexing model from the issue text.