Use std::function for the global error handler

[SpriteOvO]

// error C2664: 'void spdlog::set_error_handler(void (__cdecl *)(const std::string &))': cannot convert argument 1 from '<lambda_1>' to 'void (__cdecl *)(const std::string &)'
//
spdlog::set_error_handler([&](const std::string &message) { /* ... */ });
spdlog::set_error_handler([=](const std::string &message) { /* ... */ });

The pure pointer function type error handler cannot store lambda with capture or std::bind.

I think this might be a mistake? Because the error handler in spdlog::logger is using err_handler (std::function).

This PR changes the global error handler type from a pure pointer function (void (*)(const std::string &msg)) to err_handler (std::function<void(const std::string &err_msg)>) to fix that problem.

[gabime]

Thanks @SpriteOvO . Merged.