Unify engine error to reject blocks (#9085)

* Reject if Engine::on_close_block returns error

* Unify open block behaviors

* Fix tests in ethcore

* Fix Aura tests

* Fix RPC test

* Print a warning if open block failed

* Print the actual error when closing the block

* Update comments for prepare_pending_block

* Add BlockPreparationStatus to distingish three different state after prepare_pending_block