Use clearer error for pages when SHA is outdated

The previous error message was vague and could imply the pages daemon
was outdated which is not true.

app/services/projects/update_pages_service.rb

@@ -27,7 +27,7 @@ module Projects
       @status.run!
 
       raise InvalidStateError, 'missing pages artifacts' unless build.artifacts?
-      raise InvalidStateError, 'pages are outdated' unless latest?
+      raise InvalidStateError, 'build SHA is outdated for this ref' unless latest?
 
       # Create temporary directory in which we will extract the artifacts
       make_secure_tmp_dir(tmp_path) do |archive_path|
@@ -36,7 +36,7 @@ module Projects
         # Check if we did extract public directory
         archive_public_path = File.join(archive_path, PUBLIC_DIR)
         raise InvalidStateError, 'pages miss the public folder' unless Dir.exist?(archive_public_path)
-        raise InvalidStateError, 'pages are outdated' unless latest?
+        raise InvalidStateError, 'build SHA is outdated for this ref' unless latest?
 
         deploy_page!(archive_public_path)
         success

changelogs/unreleased/descriptive_pages_error.yml

+---
+title: Use clearer error message for pages deploy job when the SHA is outdated
+merge_request: 25659
+author:
+type: other

spec/services/projects/update_pages_service_spec.rb

@@ -82,6 +82,9 @@ describe Projects::UpdatePagesService do
 
         expect(execute).not_to eq(:success)
         expect(project.pages_metadatum).not_to be_deployed
+
+        expect(deploy_status).to be_failed
+        expect(deploy_status.description).to eq('build SHA is outdated for this ref')
       end
 
       context 'when using empty file' do