`BlockManager::AcceptBlockHeader` calls `ContextualCheckBlockHeader` and logs an error in case of failure, when `ContextualCheckBlockHeader` already logged an error. The only additional information added by the `AcceptBlockHeader` log line is the block hash. This commit adds the block hash to the first log message in `ContextualCheckBlockHeader` and removes the redundant log line.
It also moves these error messages to the `BCLog::VALIDATION` category, so that they don't unconditionnaly pollute the logs. There messages are very common because of the headers sent by Bitcoin Cash and Bitcoin SV nodes.
Note that the only other function calling `ContextualCheckBlockHeader` is `TestBlockValidity`, which is used by mining related code (`CreateNewBlock`, `generateblock` and `getblocktemplate`). `TestBlockValidity` will still unconditionnaly log an error message when `ContextualCheckBlockHeader` returns `false`.