4.0: reverse conditions in the `atomic` callback of 4.0

Right now, you say the condition whereby the error should be raised. In 4.0, this condition should be flipped to be the condition where by the error should *not* be raised. This will mirror the upcoming support of `validate expr(thing_that_must_be_true)`.

Start by locating the `atomic` callback implementation and the existing or upcoming `validate expr` support referenced in the issue. Confirm how the callback currently interprets its condition, then verify that the 4.0 behavior mirrors the stated `validate expr(thing_that_must_be_true)` semantics.

Written by the indexing model from the issue text.