Add error handling for inline blame GraphQL query

What does this MR do?

Adds error handling to the inline blame GraphQL query used in two components:

• simple_viewer.vue
• source_viewer.vue

Previously, if the blame query failed, it would fail silently without any user feedback. Now users see a clear error message when the blame information cannot be loaded.

Issue: #568003

Changes

• Added try-catch blocks around the blame GraphQL query in both components
• Display flash message "Failed to load blame information" when query fails
• Added test coverage for error scenarios
• Uses GitLab's standard createAlert

Testing Steps

Manual Testing

1. Enable inline_blame feature flag
2. Navigate to any file in the repository
3. Click the "Blame" button to enable blame view
4. To test error handling:
   • Open browser dev tools → Network tab
   • Enable "Offline" mode or block GraphQL requests or add ignore_revs=true param if ignore revs file not available
   • Click "Blame" button again
   • Verify that a flash message appears: "Failed to load blame information"
   • Disable offline mode and verify blame works normally again

Screenshots