Turns out that RtlCaptureStackBackTrace is actually just doing FP (ebp)
unwinding under the hood, making this logic completely redundant with
our own FP-walking implementation; see added comment for details.
The downside of this commit is that more precise errors are no longer
propagated up. However, these errors were pretty useless in isolation
due to them having no context; and regardless, we intentionally swallow
most of them in `std.debug` anyway. Therefore, this is better in
practice, because it allows `std.debug` to give slightly more useful
warnings when handling errors. This commit does that for unwind errors,
for instance, which differentiate between the unwind info being corrupt
vs missing vs inaccessible vs unsupported.
A better solution would be to also include more detailed information via
the diagnostics pattern, but this commit is an incremental improvement.
