Fix error handling for Bitbucket Server importer

We no longer use gitlab-shell to clone and fetch, so update the
exception for Gitaly gRPC calls.

lib/gitlab/bitbucket_server_import/importer.rb

@@ -145,7 +145,7 @@ module Gitlab
         project.repository.fetch_as_mirror(project.import_url, refmap: self.class.refmap)
 
         log_info(stage: 'import_repository', message: 'finished import')
-      rescue Gitlab::Shell::Error => e
+      rescue ::Gitlab::Git::CommandError => e
         Gitlab::ErrorTracking.log_exception(
           e,
           stage: 'import_repository', message: 'failed import', error: e.message

spec/lib/gitlab/bitbucket_server_import/importer_spec.rb

@@ -43,10 +43,10 @@ RSpec.describe Gitlab::BitbucketServerImport::Importer do
       subject.execute
     end
 
-    it 'raises a Gitlab::Shell exception in the fetch' do
-      expect(project.repository).to receive(:fetch_as_mirror).and_raise(Gitlab::Shell::Error)
+    it 'raises a Gitlab::Git::CommandError in the fetch' do
+      expect(project.repository).to receive(:fetch_as_mirror).and_raise(::Gitlab::Git::CommandError)
 
-      expect { subject.execute }.to raise_error(Gitlab::Shell::Error)
+      expect { subject.execute }.to raise_error(::Gitlab::Git::CommandError)
     end
 
     it 'raises an unhandled exception in the fetch' do