Fixed error page display with enabled multistore

[gibkigonzo]

Related issues

closes #3890

Short description and why it's useful

We need to keep /page-not-found as last one in router/index.js because it has wild card and routes are processed in order, so it can match /error. Second problem is that we set storeCode here https://github.com/DivanteLtd/vue-storefront/blob/develop/core/lib/multistore.ts#L59 then we register user module where state is overridden by default one https://github.com/DivanteLtd/vue-storefront/blob/develop/core/modules/user/index.ts#L10 . I was thinking about doing something like this in core/modules/user/index.ts

store.registerModule('user', {...userStore, state: {...userStore.state, ...store.state.user}})

But if storeView is not cleared in some moment then I think we can just use storeView.storeCode instead of user.current_storecode. https://github.com/DivanteLtd/vue-storefront/pull/3920/files#diff-009121668441dc0628ed96b5d5c41d64R22

Screenshots of visual changes before/after (if there are any)

Which environment this relates to

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• Test version (https://test.storefrontcloud.io) - this is a new feature or improvement for Vue Storefront. I've created branch from develop branch and want to merge it back to develop
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