log: use error level for critical log messages

As per doc/developer-notes#logging, LogError should be used for
severe problems that require the node to shut down.

Co-Authored-By: stickies-v <stickies-v@protonmail.com>

src/node/interfaces.cpp

@@ -123,7 +123,7 @@ public:
     void startShutdown() override
     {
         if (!(*Assert(Assert(m_context)->shutdown))()) {
-            LogPrintf("Error: failed to send shutdown signal\n");
+            LogError("Failed to send shutdown signal\n");
         }
         // Stop RPC for clean shutdown if any of waitfor* commands is executed.
         if (args().GetBoolArg("-server", false)) {