refactor: Move LoadGenesisBlock to ChainstateManager

The function does not need anything from any chainstate, so it should
not sit in the Chainstate class.

Also, mark it [[nodiscard]], and the one place that ignores the return
value with (void).

Also, change the error log strings to not include the __func__, which is
redundant with -logsourcelocations. This is not a refactor, but this log
is only for debugging extremely rare errors.

src/node/blockstorage.cpp

@@ -1305,7 +1305,7 @@ void ImportBlocks(ChainstateManager& chainman, std::span<const fs::path> import_
         chainman.m_blockman.m_blockfiles_indexed = true;
         LogInfo("Reindexing finished");
         // To avoid ending up in a situation without genesis block, re-try initializing (no-op if reindexing worked):
-        chainman.ActiveChainstate().LoadGenesisBlock();
+        (void)chainman.LoadGenesisBlock();
     }
 
     // -loadblock=

src/node/chainstate.cpp

@@ -62,7 +62,7 @@ static ChainstateLoadResult CompleteChainstateInitialization(
     // If we're not mid-reindex (based on disk + args), add a genesis block on disk
     // (otherwise we use the one already on disk).
     // This is called again in ImportBlocks after the reindex completes.
-    if (chainman.m_blockman.m_blockfiles_indexed && !chainman.ActiveChainstate().LoadGenesisBlock()) {
+    if (chainman.m_blockman.m_blockfiles_indexed && !chainman.LoadGenesisBlock()) {
         return {ChainstateLoadStatus::FAILURE, _("Error initializing block database")};
     }