Improve the error message when a table does not have secondary index sort key annotation

[This line of code](https://github.com/cashapp/tempest/blob/master/tempest2/src/main/kotlin/app/cash/tempest2/internal/V2.kt#L82) currently throws a NPE instead of suggesting the developer to add a `@DynamoDbSecondaryIndexSortKey` annotation

Start at tempest2/src/main/kotlin/app/cash/tempest2/internal/V2.kt line 82, where the missing secondary-index sort-key annotation currently causes an NPE. Trace the surrounding lookup to determine the expected missing-annotation case. Done means the error clearly suggests adding @DynamoDbSecondaryIndexSortKey instead of throwing an NPE.

Written by the indexing model from the issue text.