MissingSchemaError instead of 404

Hi,

I tried mers in my project: `app.use('/rest', mers({..}).rest())`

...But when I visit `/rest/none` where `none` is an unknown schema, I get error:

```
   500 MissingSchemaError: Schema hasn't been registered for model "undefined". Use mongoose.model(name, schema)
```

It's strange to see `500` here, because "non-existant" has the code of `404`. 
