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

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

-mutation ($projectPath: ID!, $iid: String!, $healthStatus: HealthStatus) {
-  updateIssue(input: { projectPath: $projectPath, iid: $iid, healthStatus: $healthStatus}) {
+mutation($projectPath: ID!, $iid: String!, $healthStatus: HealthStatus) {
+  updateIssue(input: { projectPath: $projectPath, iid: $iid, healthStatus: $healthStatus }) {
     issue {
       healthStatus
     }
+    errors
   }
 }

app/assets/javascripts/snippets/components/snippet_header.vue

@@ -137,7 +137,10 @@ export default {
           mutation: DeleteSnippetMutation,
           variables: { id: this.snippet.id },
         })
-        .then(() => {
+        .then(({ data }) => {
+          if (data?.destroySnippet?.errors) {
+            throw new Error(data?.destroySnippet?.errors[0]);
+          }
           this.isDeleting = false;
           this.errorMessage = undefined;
           this.closeDeleteModal();

ee/app/assets/javascripts/sidebar/sidebar_mediator.js

@@ -33,7 +33,12 @@ export default class SidebarMediator extends CESidebarMediator {
     this.store.setFetchingState('status', true);
     return this.service
       .updateWithGraphQl(updateStatusMutation, { healthStatus })
-      .then(({ data }) => this.store.setStatus(data?.updateIssue?.issue?.healthStatus))
+      .then(({ data }) => {
+        if (data?.updateIssue?.errors?.length > 0) {
+          throw data.updateIssue.errors[0];
+        }
+        this.store.setStatus(data?.updateIssue?.issue?.healthStatus);
+      })
       .catch(error => {
         throw error;
       })

spec/frontend/snippets/components/snippet_header_spec.js

@@ -32,7 +32,7 @@ describe('Snippet header component', () => {
   const errorMsg = 'Foo bar';
   const err = { message: errorMsg };
 
-  const resolveMutate = jest.fn(() => Promise.resolve());
+  const resolveMutate = jest.fn(() => Promise.resolve({ data: {} }));
   const rejectMutation = jest.fn(() => Promise.reject(err));
 
   const mutationTypes = {