They are optional. If the texture isn't found, it isn't replaced. Pretty straightforward. Especially since this particular texture replacement is only applicable on an MVP model.
However, the error message definitely needs to be commensurate with the issue. It's meant to be a notification to modders, saying, "By the way, the texture you're assuming is there really isn't, so I'm not going to use it. Just FYI in case you misspelled something." So it should be a line in the debug log rather than an in-your-face warning.
And what does a bucket have to do with anything?