Display an error when failed to load diff metadata

app/assets/javascripts/mr_notes/stores/actions.js

app/assets/javascripts/mr_notes/stores/modules/index.js

app/assets/javascripts/mr_notes/stores/mutation_types.js

app/assets/javascripts/mr_notes/stores/mutations.js

app/assets/javascripts/notes/components/note_body.vue

...ripts/vue_shared/components/markdown/apply_suggestion.vue

...cripts/vue_shared/components/markdown/suggestion_diff.vue

...vue_shared/components/markdown/suggestion_diff_header.vue

...avascripts/vue_shared/components/markdown/suggestions.vue

locale/gitlab.pot

...tures/merge_request/user_suggests_changes_on_diff_spec.rb

spec/frontend/mr_notes/stores/actions_spec.js

...d/vue_shared/components/markdown/apply_suggestion_spec.js