Fix misleading error message when Babe owner keys != 1

Solves #191 (closed) by having the error message cover both the "too many keys" and the "zero keys" situations.

requested review from @HugoTrentesaux

assigned to @txels

changed the description

added RN-binary label

changed milestone to %runtime-802

mentioned in commit 703390b3

merged

Thanks ;)

mentioned in commit 1fcc2f7a