module: Tweak backdate-warning-turned error (#58651)
This makes two changes to the backdate-warning-turned-error (#58266):
1. Fix a bug where the error would only trigger the first time. We do
only want to print once per process, but of course, we do want to error
every time if enabled.
2. If we are in speculative execution context (generators and
speculatively run functions during inference), always use the
UndefVarError. Effects from these functions are not supposed to be
observable, and it's very confusing if the printed warning goes away
when depwarns are enabled. This is marginally more breaking, but the
burden is on generated function authors (which already have to be
world-age aware and are somewhat more regularly broken) and is
consistent with other things that are stronger errors in pure context.
Fixes #58648
