improvements to importing multiple assets

`import_images()`, `import_meshes()` and `import_landmark_files()` have no all grown a `verbose` flag. Set it to `True` to get nice dynamic printing of the progress of asset importing (@nontas - proud? ;) ). This PR updates the `load_database` function to use this newly supported flag, simplifying the database loading code.

We now also raise a `ValueError` if the glob the user gives provides no assets. Tests are added for this behaviour.