node/chainstate: Decouple from concept of uiInterface

...instead allow the caller to optionally pass in callbacks which are
triggered for certain events.

Behaviour change: The string "Verifying blocks..." was previously
printed for each chainstate in chainman which did not have an
effectively empty coinsview, now it will be printed once unconditionally
before we call VerifyLoadedChain.

src/init.cpp

@@ -1427,7 +1427,12 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
                                  fReindexChainState,
                                  nBlockTreeDBCache,
                                  nCoinDBCache,
-                                 nCoinCacheUsage);
+                                 nCoinCacheUsage,
+                                 []() {
+                                     uiInterface.ThreadSafeMessageBox(
+                                         _("Error reading from database, shutting down."),
+                                         "", CClientUIInterface::MSG_ERROR);
+                                 });
         if (rv.has_value()) {
             switch (rv.value()) {
             case ChainstateLoadingError::ERROR_LOADING_BLOCK_DB:
@@ -1463,6 +1468,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
                 break;
             }
         } else {
+            uiInterface.InitMessage(_("Verifying blocks…").translated);
             auto rv2 = VerifyLoadedChainstate(chainman,
                                               fReset,
                                               fReindexChainState,