Raised when the vault reports that required encryption-context keys were not supplied (ADR-0001 decision 6, acceptance amendment 2).
Kept separate from Encryptor.Ecto.DecryptError on purpose. The vault's
{:missing_required_context_keys, _} is the one context failure that is
distinguishable in its :reason, and what it reports is a host
misconfiguration - a field pointed at a vault whose profile requires a pair
this declaration does not supply. That is a deployment defect to fix, not an
integrity event to investigate, and the two should not page the same person
with the same words.
:missing_keys carries the key names the vault named. They are names, not
values.