Does not handle case where iframe fails to load

[Hengjie]

TypeError: Cannot read property 'contentWindow' of undefined
    at buildMessage (xdLocalStorage.js:53)
    at window.xdLocalStorage.window.xdLocalStorage.getItem (xdLocalStorage.js:112)
    at ng-xdLocalStorage.js:29

If the iframe fails to load for whatever reason, it should be handled gracefully with a rejection of promise.

[loter]

any chance to get it fixed? :) still experiencing this issue when using dist/scripts/ng-xdLocalStorage.min.js

[ofirdagan]

@loter - you can continue on prull request #19 and I'll merge. Currently it has conflicts..