fix($compile): throw an exception when an attribute is unassigned

Documentation states that NON_ASSIGNABLE_MODEL_EXPRESSION exception should be thrown if
a two-way attribute doesn't exist. This commit adds a simple test and throws an error
if the property is missing. Note that the documentation should probably be updated
and a different exception should be thrown using minErr.

This commit refers to issue #6060
You don't need the ```else``` statement since the ``ìf``` returns.

So that you could do:

```
if (optional) return;
throw new Error(...);
```