Fail gracefully when security settings prevent access to sessionStorage

[masonicboom]

This can happen (at least) in iOS 9 Mobile Safari, when "Block Cookies" is set to "Always Block" (angular-translate/angular-translate#629 suggests this has been true since iOS 7.1.2).

[taion]

This is the same as #99. I think we just want to handle everything in #104.

[mjackson]

This is a different error, @taion. #104 is about handling errors when the quota is actually exceeded. This PR is about handling errors when cookies are disabled in a specific environment.

[taion]

It's not the same as #104, but it's a more targeted version of the same check as #99, geared toward the same thing. I guess we'd merge these separately and resolve the conflicts later?

[mjackson]

Thanks, @masonicboom!

@taion Yes, we'll resolve and merge #104 separately.