Error message better when trying to use an invalid model prop.

When using an invalid property while querying the model, like so:

```javascript
geddy.model.MyModel.find({ bad_prop: 'some_value' }, function () {
// ...
});
```

Geddy model would throw this exception:

```
TypeError: Cannot read property 'datatype' of undefined at _createComparison
(/usr/local/lib/node_modules/geddy/node_modules/model/lib/query/query.js:232:27) at _createOperation
(/usr/local/lib/node_modules/geddy/node_modules/model/lib/query/query.js:175:43) at Query.initialize
(/usr/local/lib/node_modules/geddy/node_modules/model/lib/query/query.js:345:40) at Query
(/usr/local/lib/node_modules/geddy/node_modules/model/lib/query/query.js:12:19) at Function.obj.all
(/usr/local/lib/node_modules/geddy/node_modules/model/lib/index.js:378:15) at show
(/Users/my_username/src/my_app/app/controllers/my_controller.js:12:26) at callback
(/usr/local/lib/node_modules/geddy/lib/controller/base_controller.js:282:22) at controller.BaseController._handleAction
(/usr/local/lib/node_modules/geddy/lib/controller/base_controller.js:295:7) at cb [as callback]
(/usr/local/lib/node_modules/geddy/lib/app/index.js:258:36) at async.Initializer.complete
(/usr/local/lib/node_modules/geddy/node_modules/utilities/lib/async.js:286:10)
```

This is pretty unhelpful – I have changed the error to be more informative to the end-user:

```
Error: The property "bad_prop" is not a valid property on the MyModel model. at _createComparison
```