* 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
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