Added better error message for poorly formed frontmatters in archtypes

Ran into an issue where the theme I was using (purehugo) had an empty archetype `themes/purehugo/archetypes/default.md`. The produced error message was a bit cryptic:

```
ERROR: 2014/11/02 Unable to Cast <nil> to map[string]interface{}
```

This makes it a bit more explicit:
```
ERROR: 2014/11/02 Error processing archetype file: /path/to/project/themes/purehugo/archetypes/default.md
```