Return a 422 error for Changelog::Error exception

* Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/351520
* Sentry error: https://sentry.gitlab.net/gitlab/gitlabcom/issues/3190373

Changelog: fixed

lib/api/repositories.rb

@@ -248,6 +248,8 @@ module API
         changelog = service.execute(commit_to_changelog: false)
 
         present changelog, with: Entities::Changelog
+      rescue Gitlab::Changelog::Error => ex
+        render_api_error!("Failed to generate the changelog: #{ex.message}", 422)
       end
 
       desc 'Generates a changelog section for a release and commits it in a changelog file' do

spec/requests/api/repositories_spec.rb

@@ -783,6 +783,13 @@ RSpec.describe API::Repositories do
       expect(response).to have_gitlab_http_status(:ok)
       expect(json_response['notes']).to be_present
     end
+
+    context 'when previous tag version does not exist' do
+      it_behaves_like '422 response' do
+        let(:request) { get api("/projects/#{project.id}/repository/changelog", user), params: { version: 'v0.0.0' } }
+        let(:message) { 'Failed to generate the changelog: The commit start range is unspecified, and no previous tag could be found to use instead' }
+      end
+    end
   end
 
   describe 'POST /projects/:id/repository/changelog' do