Show errors in flash if DAST on-demand scan fails

ee/app/assets/javascripts/on_demand_scans/components/on_demand_scans_form.vue

 <script>
 import * as Sentry from '@sentry/browser';
-import { s__ } from '~/locale';
+import { s__, sprintf } from '~/locale';
 import createFlash from '~/flash';
 import { isAbsolute, redirectTo } from '~/lib/utils/url_utility';
 import {
@@ -99,8 +99,17 @@ export default {
           mutation: runDastScanMutation,
           variables: this.formData,
         })
-        .then(({ data: { runDastScan: { pipelineUrl } } }) => {
-          redirectTo(pipelineUrl);
+        .then(({ data: { runDastScan: { pipelineUrl, errors } } }) => {
+          if (errors?.length) {
+            createFlash(
+              sprintf(s__('OnDemandScans|Could not run the scan: %{backendErrorMessage}'), {
+                backendErrorMessage: errors.join(', '),
+              }),
+            );
+            this.loading = false;
+          } else {
+            redirectTo(pipelineUrl);
+          }
         })
         .catch(e => {
           Sentry.captureException(e);

ee/app/assets/javascripts/on_demand_scans/graphql/run_dast_scan.mutation.graphql

@@ -13,5 +13,6 @@ mutation runDastScan(
     }
   ) {
     pipelineUrl
+    errors
   }
 }

ee/spec/frontend/on_demand_scans/components/on_demand_scans_form_spec.js

@@ -194,7 +194,7 @@ describe('OnDemandScansApp', () => {
       beforeEach(async () => {
         jest
           .spyOn(wrapper.vm.$apollo, 'mutate')
-          .mockResolvedValue({ data: { runDastScan: { pipelineUrl } } });
+          .mockResolvedValue({ data: { runDastScan: { pipelineUrl, errors: [] } } });
         const input = findTargetUrlInput();
         input.vm.$emit('input', targetUrl);
         submitForm();
@@ -221,7 +221,7 @@ describe('OnDemandScansApp', () => {
       });
     });
 
-    describe('on error', () => {
+    describe('on top-level error', () => {
       beforeEach(async () => {
         jest.spyOn(wrapper.vm.$apollo, 'mutate').mockRejectedValue();
         const input = findTargetUrlInput();
@@ -237,5 +237,25 @@ describe('OnDemandScansApp', () => {
         expect(createFlash).toHaveBeenCalledWith('Could not run the scan. Please try again.');
       });
     });
+
+    describe('on errors as data', () => {
+      beforeEach(async () => {
+        const errors = ['A', 'B', 'C'];
+        jest
+          .spyOn(wrapper.vm.$apollo, 'mutate')
+          .mockResolvedValue({ data: { runDastScan: { pipelineUrl: null, errors } } });
+        const input = findTargetUrlInput();
+        input.vm.$emit('input', targetUrl);
+        submitForm();
+      });
+
+      it('resets loading state', () => {
+        expect(wrapper.vm.loading).toBe(false);
+      });
+
+      it('shows an error flash', () => {
+        expect(createFlash).toHaveBeenCalledWith('Could not run the scan: A, B, C');
+      });
+    });
   });
 });

locale/gitlab.pot

@@ -16388,6 +16388,9 @@ msgstr ""
 msgid "OnDemandScans|Could not run the scan. Please try again."
 msgstr ""
 
+msgid "OnDemandScans|Could not run the scan: %{backendErrorMessage}"
+msgstr ""
+
 msgid "OnDemandScans|Create new DAST scan"
 msgstr ""