Improve error reporting

[lubomir]

When trying to load a compose with productmd.compose.Compose class, if some file is not found, an AttributeError gets reported. Because of the lazy loading, there is no reliable way to even catch such exceptions.

[lubomir]

Pull request #36 adds explicit exception with a descriptive error message. It is still raised at a random point in time when some attribute of Compose class is first accessed. Changing that would be a bigger effort, as currently there may be people relying on the ability to load only partial data.