Return a readable error message for epic assignment failure

ee/app/graphql/mutations/issues/set_epic.rb

@@ -18,8 +18,12 @@ module Mutations
 
         authorize_admin_rights!(epic)
 
-        ::Issues::UpdateService.new(project: project, current_user: current_user, params: { epic: epic })
-          .execute(issue)
+        begin
+          ::Issues::UpdateService.new(project: project, current_user: current_user, params: { epic: epic })
+            .execute(issue)
+        rescue EE::Issues::BaseService::EpicAssignmentError => error
+          issue.errors.add(:base, error.message)
+        end
 
         {
           issue: issue,

ee/app/models/epic_issue.rb

@@ -37,7 +37,7 @@ class EpicIssue < ApplicationRecord
     return unless epic && issue
 
     if epic.confidential? && !issue.confidential?
-      errors.add :issue, _('Cannot set confidential epic for a non-confidential issue')
+      errors.add :issue, _('Cannot assign a confidential epic to a non-confidential issue. Make the issue confidential and try again')
     end
   end
 end

ee/spec/graphql/mutations/issues/set_epic_spec.rb

@@ -59,6 +59,14 @@ RSpec.describe Mutations::Issues::SetEpic do
             expect(mutated_issue.epic).to eq(nil)
           end
         end
+
+        context 'when epic is confidential but issue is public' do
+          let(:epic) { create(:epic, group: group, confidential: true) }
+
+          it 'returns an error with appropriate message' do
+            expect(subject[:errors].first).to include("Cannot assign a confidential epic to a non-confidential issue. Make the issue confidential and try again")
+          end
+        end
       end
     end
   end

ee/spec/services/epic_issues/create_service_spec.rb

@@ -235,8 +235,8 @@ RSpec.describe EpicIssues::CreateService do
             expect(subject).to eq(
               status: :error,
               http_status: 422,
-              message: "#{invalid_issue1.to_reference} cannot be added: Cannot set confidential epic for a non-confidential issue. "\
-                       "#{invalid_issue2.to_reference} cannot be added: Cannot set confidential epic for a non-confidential issue"
+              message: "#{invalid_issue1.to_reference} cannot be added: Cannot assign a confidential epic to a non-confidential issue. Make the issue confidential and try again. "\
+                       "#{invalid_issue2.to_reference} cannot be added: Cannot assign a confidential epic to a non-confidential issue. Make the issue confidential and try again"
             )
           end
         end

locale/gitlab.pot

@@ -6225,6 +6225,9 @@ msgstr ""
 msgid "Cancelling Preview"
 msgstr ""
 
+msgid "Cannot assign a confidential epic to a non-confidential issue. Make the issue confidential and try again"
+msgstr ""
+
 msgid "Cannot be assigned to other projects."
 msgstr ""
 
@@ -6276,9 +6279,6 @@ msgstr ""
 msgid "Cannot refer to a group %{timebox_type} by an internal id!"
 msgstr ""
 
-msgid "Cannot set confidential epic for a non-confidential issue"
-msgstr ""
-
 msgid "Cannot show preview. For previews on sketch files, they must have the file format introduced by Sketch version 43 and above."
 msgstr ""