Improve messages for unknown reference

Currently the error message for an unknown reference is:

```
Cannot resolve symbol '...'
```

This can be improved based on where the error is.

e.g.

* In `@Mapping` we can do "Unknown property '...'"
* In `@ValueMapping` we can do "Unknown enum constant '...'"
* In `Mapping#qualifiedByName` we can do "Unknown @Named reference '...'"