The problem with the InnoDB table attribute encryption_key_id is that it is
not being persisted anywhere in InnoDB except if the table attribute
encryption is specified and is something else than encryption=default.
MDEV-17320 made it a hard error if encryption_key_id is specified to be
anything else than 1 in that case.

Ideally, we would always persist encryption_key_id in InnoDB. But, then we
would have to be prepared for the case that when encryption is being enabled
for a table whose encryption_key_id attribute refers to a non-existing key.

In MariaDB Server 10.1, our best option remains to not store anything
inside InnoDB. But, instead of returning the error that MDEV-17320
introduced, we should merely issue a warning that the specified
encryption_key_id is going to be ignored if encryption=default.

To improve the situation a little more, we will issue a warning if
SET [GLOBAL|SESSION] innodb_default_encryption_key_id is being set
to something that does not refer to an available encryption key.

Starting with MariaDB Server 10.2, thanks to MDEV-5800, we could open the
table definition from InnoDB side when the encryption is being enabled,
and actually fix the root cause of what was reported in MDEV-17320.