Fixed Grunt shutdown on unhandled error

Since changed `async.forEach` to `async.forEachSeries`. There starts an infinite loop when compilation error happens, but the infinite loop ends right on the first accessing of `f.dest` beacuse `f` is `undefined`.

`nextFileObj(false);` in `forEachSeries` does not end the loop (opposite to `forEach` behavior), instead it continues it.

Passing `err` object instead of `false` does well. Automated compilation with watch systems cannot work well with of this bug.