refactor: Use untranslated error message in ActivateSnapshot

The message is not exposed in the GUI or another translated context, so
translating it is useless for now.

Also, fix a nit from bitcoin/bitcoin#30395 (comment)

Author: MarcoFalke

src/validation.cpp

@@ -5681,7 +5681,7 @@ util::Result<void> ChainstateManager::ActivateSnapshot(
         }
 
         if (!m_best_header || m_best_header->GetAncestor(base_blockheight) != snapshot_start_block) {
-            return util::Error{_("A forked headers-chain with more work than the chain with the snapshot base block header exists. Please proceed to sync without AssumeUtxo.")};
+            return util::Error{Untranslated("A forked headers-chain with more work than the chain with the snapshot base block header exists. Please proceed to sync without AssumeUtxo.")};
         }
 
         auto mempool{m_active_chainstate->GetMempool()};
@@ -5735,7 +5735,7 @@ util::Result<void> ChainstateManager::ActivateSnapshot(
             static_cast<size_t>(current_coinstip_cache_size * SNAPSHOT_CACHE_PERC));
     }
 
-    auto cleanup_bad_snapshot = [&](bilingual_str&& reason) EXCLUSIVE_LOCKS_REQUIRED(::cs_main) {
+    auto cleanup_bad_snapshot = [&](bilingual_str reason) EXCLUSIVE_LOCKS_REQUIRED(::cs_main) {
         this->MaybeRebalanceCaches();
 
         // PopulateAndValidateSnapshot can return (in error) before the leveldb datadir