Apollo boards - Fix some error handling

What does this MR do and why?

Fixes some error handling for Apollo boards and removes some VueX usage

Behind :apollo_boards feature flag

Screenshots or screen recordings

Before	After

How to set up and validate locally

Deleting a board requires an EE license. Ensure the deleteBoard query is set to fail, either by setting your browser offline or commenting out some query variable in the code.

1. In rails console enable the feature flag

   Feature.enable(:apollo_boards)

2. Visit a board
3. In the top left corner dropdown, select Delete board
4. Verify that the error message is displayed in the modal

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.