Docs: couple of improvements to the Vuex frontend guide

What does this MR do?

First of all, thanks for the great documentation and guide you've written for using Vuex 👍 Easy to follow and stuffed with good practises.

Here's two nitpick improvements to the following:

• set state.isAddingUser = false when request resulted in an error, as the request has finished after all
• be very explicit about vuex store being a local module to avoid confusion about a dependency named store

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• Link docs to and from the higher-level index page, plus other related docs where helpful. -- not relevant
• Apply the ~Documentation label.

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