If the user's cookie/persistent storage setting forbid access to localStorage,
catch the exception and abort the access.

Currently, attempting to use the expand/contract links at the top of the page
for structs/consts/etc. fails due to an unhandled error while accessing
localStorage, if such access is forbidden, as the exception from the failed
access propagates all the way out, interrupting the expand/contract. Instead, I
would like to degrade gracefully; the access won't happen (the collapse/expand
state won't get persisted) but the actual expanding/contracting of the item
will go on to succeed.

Fixes rust-lang#55079

…tValue()

1. Extract the tests for whether or not we have workable localStorage out into
   a helper method, so it can be more easily reused
2. Use it in getCurrentValue() too, for the same reasons, as suggested in code
   review