Unless the error makes SSP, offline, unplayable it can be ignored. If you cannot play offline please do this. Try to play offline and get the error. Now do not close the launcher or the console. Connect to the internet and click on the 'Paste my log to pastebin.com' This will add a link at the bottom of the launcher. Provide that link to us so we can see the full console log. If you are unable to either get the link or connect to the internet please copy the log to pastebin.com yourself and provide us the link that it gives you. Look
here to ensure your log does not get deleted.