mw.loader.store: Wrap script eval in try/catch

All sorts of things can happen to our localStorage entry, from
clueless users through malicious scripts and browser bugs to hard disk
failures. Let's not explode irrecoverably when the code from store
refuses to execute.

Bug: 57567
Change-Id: I1b40a82595fe37dfeeb3648f7ecfbcac01bf9079
Merge "mw.loader.store: Wrap script eval in try/catch"