Ensure all mutations have 'errors' field and a proper handler

What does this MR do?

1. Adds the errors field to all mutations
2. Provides a basic error handling for all mutations to ensure errors-as-data are handled

Affected mutations

• app/assets/javascripts/sidebar/queries/updateStatus.mutation.graphql

Affected error handlers

• ee/app/assets/javascripts/sidebar/sidebar_mediator.js
• app/assets/javascripts/snippets/components/snippet_header.vue

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Issue: #32085 (closed)