Fix Bitbucket Server importer error handler

If the Bitbucket Server importer encountered any error during a pull
request import, an `undefined local variable or method 'backtrace'`
error would previously be thrown.

1ee162b6 dropped a variable that
was needed to log a clean backtrace. We add it back now so that
error recovery will work again.

Part of https://gitlab.com/gitlab-org/gitlab/issues/197881

changelogs/unreleased/sh-add-missing-backtrace-define.yml

+---
+title: Fix Bitbucket Server importer error handler
+merge_request: 23310
+author:
+type: fixed

lib/gitlab/bitbucket_server_import/importer.rb

@@ -172,6 +172,7 @@ module Gitlab
               stage: 'import_pull_requests', iid: pull_request.iid, error: e.message
             )
 
+            backtrace = Gitlab::BacktraceCleaner.clean_backtrace(e.backtrace)
             errors << { type: :pull_request, iid: pull_request.iid, errors: e.message, backtrace: backtrace.join("\n"), raw_response: pull_request.raw }
           end
         end

spec/lib/gitlab/bitbucket_server_import/importer_spec.rb

@@ -67,6 +67,7 @@ describe Gitlab::BitbucketServerImport::Importer do
         author_email: project.owner.email,
         created_at: Time.now,
         updated_at: Time.now,
+        raw: {},
         merged?: true)
 
       allow(subject.client).to receive(:pull_requests).and_return([pull_request])
@@ -239,6 +240,13 @@ describe Gitlab::BitbucketServerImport::Importer do
       expect(notes.first.note).to start_with('*Comment on .gitmodules')
       expect(notes.second.note).to start_with('*Comment on .gitmodules')
     end
+
+    it 'reports an error if an exception is raised' do
+      allow(subject).to receive(:import_bitbucket_pull_request).and_raise(RuntimeError)
+      expect(Gitlab::ErrorTracking).to receive(:log_exception)
+
+      subject.execute
+    end
   end
 
   describe 'inaccessible branches' do