Add error handling for blame data fetching failures

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Problem

Currently, the blame functionality in the blob viewer doesn't have proper error handling when the GraphQL request to fetch blame data fails. This was identified during the review of MR !206220 (merged) which added skeleton loading states.

Current Behavior

When blame data fetching fails:

• The skeleton loader may remain visible indefinitely
• No user feedback is provided about the failure
• Users are left in an unclear state

Expected Behavior

When blame data fetching fails:

• Show an appropriate error message to the user
• Provide a way to retry the request
• Clear the loading state appropriately
• Follow GitLab's established error handling patterns

Technical Details

The issue occurs in the blame data fetching logic in:

• app/assets/javascripts/vue_shared/components/blob_viewers/simple_viewer.vue (line 132)
• Related components in the source viewer that handle blame state

Acceptance Criteria

• Add error handling for failed blame GraphQL requests
• Design appropriate error messaging (requires UX input)
• Implement retry functionality
• Ensure loading states are properly cleared on error
• Add tests for error scenarios
• Consider accessibility requirements for error states

Related

• Closes follow-up item from MR !206220 (merged)
• Related to the inline blame feature behind the inline_blame feature flag

Labels

Category:Source Code Management devopscreate groupsource code sectiondev typebug frontend UX workflowready for development