Warn when set cwd without expand

From [document](http://gruntjs.com/configuring-tasks#building-the-files-object-dynamically)

> "Set expand to true to enable the following options:"

But if I set files like

```
{
files: {
cwd: 'src',
src: '*.js',
dest: 'dist'
}
}
```

The result is wrong without any warn. If it is nessesry to use cwd with expand, then it shoud be restricted.

Start with the linked configuring-tasks documentation and reproduce the shown files configuration using cwd without expand. Trace how that configuration is handled and determine whether it should warn or be rejected; done means the invalid combination no longer silently produces the wrong result.

Written by the indexing model from the issue text.