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.
Considering that this should *not* fail like ever I would remove this if and log the assertion unconditionally as an error (console.error), Also, add a information message here (not strictly needed, would help to identify this). Something like:
console.error("Got an unexpected authentication error while signing up");