Validate TEXTURE field that texture does not have spaces in it

[tngreene]

Users with texture directives like

TEXTURE my texture.png are very surprised when it doesn't work for them. We know that the OBJ parser ignores tokens after it is satisfied but they don't. We should validate this on export, I think we used to before autodetect textures was removed.

Side note: I've decided that since we are totally focused on 2.80 and above and not on the converter, only the converter will get 249: stuck in front of it. The rest we assume are 2.80 related

• [ ] Unit test with bad name in texture fields
• [ ] Validation

Extremely simple