Maybe make payload_initializer return a Result--and handle it in the callers

[daym]

Currently, there's a few code blocks like this:

for x in xs {
   x.value()?;
}
insert_entry(..., |buf: &mut [u8]| {
   ...
   for x in xs {
      ... x.value().unwrap();
   }
}

That's because the payload_initializer (the closure above) doesn't return a Result (it doesn't return anything).

We could make it return a Result and adapt the Apcb::insert_entry caller to re-shrink the group on error again.