Add helpful error for when a code already exists

GitHub recently changed their API to prevent duplicate personal access
tokens from being generated. We should be nice about directing users
directly to the page from which they must manually revoke their token to
continue.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Ignore Gemfile.lock

Signed-off-by: Stephen Celis <stephen@stephencelis.com>