Provide a more detailed error message when a C++ type that's exposed to QML cannot be created

When exposing a C++ type to QML (with e.g. QML_ELEMENT), it's possible to run into this error message:

Element is not creatable.

This means that the type cannot be created in C++. An example of this is forgetting to implement some virtual functions in an QAbstractItemModel subclass. It's not unusual for C++ types like models to be written purely for the sake of being exposed to QML, and hence never directly constructed in C++ in the application code.

I think that the error message in this case should be more descriptive, even if a type not being creatable is valid C++ (e.g. intentionally missing a constructor or unimplemented virtual functions for classes that are intended to be subclassed). For example:

Type Foo is not creatable because QQuickFoo is an abstract type.