silent failure with unhelpful message if "dest" is not a string (e.g. an array)

let me preface this by saying it's completely my own fault, but I lost a fair bit of time looking at my configuration before realizing my problem (since at a glance it appeared my configuration matched the examples).

consider this ngtemplates configuration:

``` js
//ng-templates
ngtemplates: {
app: {
src: ['assets/views/*.html'],
dest: ['template.js']
}
```

running grunt:

```
$ grunt --verbose ngtemplates
...
Reading ...OK
Writing template.js...ERROR
Warning: Unable to write "template.js" file (Error code: undefined). Use --force to continue.

Aborted due to warnings.
```

this is an unhelpful error emitted from grunt's file.write function (which you us) and after some digging I realized that the real error was "TypeError: path must be a string" and that clued me into my mistake.

I might have expected an error message pointing out that dest had to be a string. Further the fact that it says:

```
Writing template.js
```

vs

```
Writing [ 'template.js' ]
```

Allowed me to think I was providing good data. (this is from this line from grunt.file.write)

``` js
grunt.verbose.write((nowrite ? 'Not actually writing ' : 'Writing ') + filepath + '...');
```

This is on version 0.5.1

edit: pointing out another thing where grunt's logging steered me wrong