Adding a better error message when a validation name is invalid.

When you tried to add a validator on a property that was non-
existent, like this:

```
var MyModel = function () {
  this.validatesPresent('bad_prop');
};
```

Gedddy Model would throw this error:

```
/usr/local/lib/node_modules/geddy/node_modules/model/lib/index.js:955
    reg[this.name].properties[name].validations[condition] =
                                   ^
TypeError: Cannot read property 'validations' of undefined
```

Which is not terribly helpful. When you add a validation for a
non-existent property now, it will throw this error:

```
Error: Validation cannot be added for "bad_prop": property does
not exist on the MyModel model.
```
Merge pull request #171 from JohnPostlethwait/validation-error

Adding a better error message when a validation name is invalid.