pmbootstrap config mirror.pmaports: can't set new repository

I've triggered the following command while developing bpo code:

$ pmbootstrap config mirrors.pmaports ""
[20:29:14] ERROR: type object 'Config' has no attribute 'mirrors.pmaports'

The error message doesn't make sense. I guess we should detect this and write a different error message instead, that this is invalid, and users should set it to "none" if they want to unset it.