Currently, setting a file input element's value property in a content
process creates a File object that seems valid but causes form
submission to fail -- inexplicably so, from the user's point of view.
The next patch will make that throw, so this patch prepares SessionStore
by making it catch that exception and leave that element unrestored (but
continue with the rest of the form). The user would already have needed
to manually re-pick the file -- until bug 1122855 is fixed to make
SessionStore properly e10s-enabled -- but this makes that more obvious.
