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
We prefer to use `minErr` when throwing errors... we may need to create a new error code.

```js
throw $compileMinErr('reqattr',
                      "Attribute '{0}' used with directive '{1}' is not optional!",
                      attrName, newIsolateScopeDirective.name);
```