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.
Move this (and the following debug lines) to line 193 (the onError handler for the signup). We don't really need to parse the assertion and log this unless there's an actual error.