Send rejects and apply DoS scoring for errors in direct block validation.

75f51f2a introduced asynchronous processing for blocks, where reject messages
and DoS scoring could be applied outside of ProcessBlock, because block
validation may happen later.

However, some types of errors are still detected immediately (in particular,
CheckBlock violations), which need acting after ProcessBlock returns.
Merge pull request #4471

40f5cb8 Send rejects and apply DoS scoring for errors in direct block validation. (Pieter Wuille)