Instead of simply removing holes from vectors or lists when converting
from Val to Broker format, error out as the receiver has no chance to
reconstruct where the hole might have been.
We could encode holes with broker::none, but this will put unnecessary
burden on language bindings and users due to the potential optionality.
Think a std::vector<uint64_t> that technically needs to be a
std::vector<std::optional<uint64_t>> to represent optional elements
properly.
Closes#3045
