Texture lookup doesn't error

[Steve Audia ETC]

When you lookup a resource of any time, it runs through a lua fetch of the
object itself and then errors when/if the ID is malformed. However, texture
lookups never go through a luaFetch and therefore never get a chance to send
out that error. Instead they fail silently, setting the texture on an object to
nothing and getTexture just blindly returning you the string you set it to.

So we should probably at least make this send an error back, even if it doesn't
fail hard at the time.