Fail loading the 3MF file if no meshes were found

This way we indicate loading the 3MF file failed instead of indicating
success but not actually loading anything.

CURA-236

plugins/3MFReader/ThreeMFReader.py

@@ -42,6 +42,10 @@ class ThreeMFReader(MeshReader):
 
                 # There can be multiple objects, try to load all of them.
                 objects = root.findall("./3mf:resources/3mf:object", self._namespaces)
+                if len(objects) == 0:
+                    Logger.log("w", "No objects found in 3MF file %s, either the file is corrupt or you are using an outdated format", file_name)
+                    return None
+
                 for object in objects:
                     mesh = MeshData()
                     node = SceneNode()