Error handling and paths for OBJ and MTL

[benc-uk]

Check what happens when

• [x] - OBJ can't be found
• [x] - OBJ references a MTL file can't be found
• [x] - OBJ refers to a MTL material that doesn't exist
• [x] - MTL references texture can't be found
• [x] - Try to load a model that doesn't exist
• [x] - Try to instance a model that doesn't exist

[benc-uk]

OBJ and instance handling throws errors and are not suppressed or replaced with defaults, leaving this behavior as no better alternative exists

Missing MTL and textures are handled by fallbacks to default materials and placeholder textures and don't result in errors