* aura: only report after checking for repeated skipped primaries

* aura: refactor duplicate code for getting epoch validator set

* aura: verify_external: report on validator set contract instance

* aura: use correct validator set epoch number when reporting

* aura: use epoch set when verifying blocks

* aura: report skipped primaries when generating seal

* aura: handle immediate transitions

* aura: don't report skipped steps from genesis to first block

* aura: fix reporting test

* aura: refactor duplicate code to handle immediate_transitions

* aura: let reporting fail on verify_block_basic

* aura: add comment about possible failure of reporting