Bug 1096824 - [Loop] Sign up requests might be unauthorized due to assertions expired

See bug https://bugzilla.mozilla.org/show_bug.cgi?id=1096824. Action happens at https://github.com/jaoo/firefoxos-loop-client/tree/1096824.
"Sorry, due to problems on our side your account session has expired. Please try logging in again. If the problem persist, you might need to log out from Firefox Accounts on the Settings App."

That is, first offer the option to log in and see if that fixes the problem, and only if the problem persists he should log off.