Move window.sessionStorage into try catch block.

In Chrome, if you change your Settings->Privacy->Content Settings->Cookies to
"Block sites from setting any data" then attempt to load hopScotch Chrome will throw the error "SecurityError: An attempt was made to break through the security policy of the user agent." 

This is reproducible on your demo site as well.

This moves accessing window.sessionStorage into try catch block to prevent Chrome and possibly other browsers from throwing an error.