Track and print names of users who can't be added to Github teams at end of sync

Currently, when a user fails to be added, the script logs the error and continues processing without any additional actions. To better manage these errors, we should collect such problems and print the affected users at the end of the script. This would make tracking down users whos EF GitHub handle no longer correspond to GitHub accounts easier, as well as monitor for any other issue with adding users in the sync process.