validation: Stricter assumeutxo error handling in LoadChainstate

Make LoadChainstate return an explicit error when snapshot validation succeeds,
but there is an error trying to replace the background chainstate with the
snapshot chainstate. Previously in this case LoadChainstate would trigger a
shutdown and return INTERRUPTED, now it will return an actual error code.

There's no real change to behavior other than error message being formatted a
little differently.

Motivation for this change is to replace error handling via callbacks with
error handling via return value ahead of
https://github.com/bitcoin/bitcoin/pull/27861

src/init.cpp

@@ -1530,7 +1530,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
             }
         }
 
-        if (status == node::ChainstateLoadStatus::FAILURE_INCOMPATIBLE_DB || status == node::ChainstateLoadStatus::FAILURE_INSUFFICIENT_DBCACHE) {
+        if (status == node::ChainstateLoadStatus::FAILURE_FATAL || status == node::ChainstateLoadStatus::FAILURE_INCOMPATIBLE_DB || status == node::ChainstateLoadStatus::FAILURE_INSUFFICIENT_DBCACHE) {
             return InitError(error);
         }