Improve exception message when trying to read list property as scalar about happly

[christian-rauch]

Ah, never mind. This is a list property and I tried to read it as a scalar property.

The correct call would be:

tracks.getElement("track").getListProperty<uint32_t>("vertex_index");

Maybe the exception should say something about list and scalar properties and not about the scalar type conversion.

[nmwsharp]

Thanks for filing! I'll convert this to a TODO to add a better error message.