This adds block reward contract config to ethash. A new config `blockRewardContractCode` is also added to both Aura and ethash. When specified, it will execute the code directly and overrides any `blockRewardContractAddress` config. Having this `blockRewardContractCode` config allows chains to deploy hard fork by simply replacing the current config value, without the need from us to support any `multi` block reward scheme.
* 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
