The storage reported something no other error describes. Log it and fail.
Adapters return it for storage responses nobody has mapped yet, with the storage's own term in :reason. Please
report it, so it can be mapped to one of the other errors.
Summary
Types
@type t() :: %Fil.UnknownError{ __exception__: term(), disk: Fil.Disk.t() | nil, op: Fil.Op.name() | nil, path: String.t() | nil, reason: term() }