Fix custom path finding and updating

This fixes the problem where when using the `theme` option it had to be relative to the `grunt-aglio` node module instead of just relative to the current project's `Gruntfile`.

For example, you would have to do something like:

```
theme: '../../../../../app/docs/theme/cp-flatly-multi'
```
instead of just

```
theme: 'app/docs/theme/cp-flatly-multi'
```

In addition, it also fixes the problem where even when you provided your custom template via a relative path to the `grunt-aglio` Gruntfile, it would still print an error saying it reverted to the `default` theme, even though it actually worked and didn't properly revert to the `default` theme. With my fix, if the custom template is not found it properly prints the error and reverts to `default`. If it is found, everything proceeds as expected.