[App/ogs] Add spdlog error handler.

Helps with identification of wrong output types etc.

Applications/CLI/ogs.cpp

@@ -123,6 +123,10 @@ int main(int argc, char* argv[])
 
     BaseLib::setConsoleLogLevel(log_level_arg.getValue());
     spdlog::set_pattern("%^%l:%$ %v");
+    spdlog::set_error_handler([](const std::string& msg) {
+        std::cerr << "spdlog error: " << msg << std::endl;
+        OGS_FATAL("spdlog logger error occured.");
+    });
 
     INFO("This is OpenGeoSys-6 version {:s}.",
          GitInfoLib::GitInfo::ogs_version);