MAINT-7482 - continue running with a warning on receipt of unknown message

indra/llmessage/lltemplatemessagereader.cpp

@@ -499,9 +499,10 @@ BOOL LLTemplateMessageReader::decodeTemplate(
 	}
 	else
 	{
-		LL_WARNS() << "Message #" << std::hex << num << std::dec
-			<< " received but not registered!" << LL_ENDL;
-		gMessageSystem->callExceptionFunc(MX_UNREGISTERED_MESSAGE);
+        // MAINT-7482 - make viewer more tolerant of unknown messages.
+		LL_WARNS_ONCE() << "Message #" << std::hex << num << std::dec
+                        << " received but not registered!" << LL_ENDL;
+		//gMessageSystem->callExceptionFunc(MX_UNREGISTERED_MESSAGE);
 		return(FALSE);
 	}