Add an ability to fail the build if property was not resolved

[just-boris]

This is a proposal to add an option that brings back the behavior removed in https://github.com/postcss/postcss-custom-properties/commit/833687a4bc656e38ac09d04938ac2d34a02b71dc

It is useful sometimes, for example, it gives confidence that if CI is green, all custom properties have been resolved.

What do you think about it? I can contribute this feature if it makes sense.