Fix import handling errors in Bitbucket Server importer

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20724 removed
`Gitlab::Git::RepositoryMirroring::RemoteError`, but this was still
being handled in the Bitbucket Server importer and only encountered if
some exception other than `Gitlab::Shell::Error` were thrown.

changelogs/unreleased/sh-remove-bitbucket-mirror-constant.yml

+---
+title: Fix import handling errors in Bitbucket Server importer
+merge_request: 24499
+author:
+type: fixed

lib/gitlab/bitbucket_server_import/importer.rb

@@ -132,7 +132,7 @@ module Gitlab
         project.repository.fetch_as_mirror(project.import_url, refmap: self.class.refmap, remote_name: REMOTE_NAME)
 
         log_info(stage: 'import_repository', message: 'finished import')
-      rescue Gitlab::Shell::Error, Gitlab::Git::RepositoryMirroring::RemoteError => e
+      rescue Gitlab::Shell::Error => e
         log_error(stage: 'import_repository', message: 'failed import', error: e.message)
 
         # Expire cache to prevent scenarios such as:
@@ -140,7 +140,7 @@ module Gitlab
         # 2. Retried import, repo is broken or not imported but +exists?+ still returns true
         project.repository.expire_content_cache if project.repository_exists?
 
-        raise e.message
+        raise
       end
 
       # Bitbucket Server keeps tracks of references for open pull requests in

spec/lib/gitlab/bitbucket_server_import/importer_spec.rb

@@ -21,12 +21,9 @@ describe Gitlab::BitbucketServerImport::Importer do
   end
 
   describe '#import_repository' do
-    before do
+    it 'adds a remote' do
       expect(subject).to receive(:import_pull_requests)
       expect(subject).to receive(:delete_temp_branches)
-    end
-
-    it 'adds a remote' do
       expect(project.repository).to receive(:fetch_as_mirror)
                                      .with('http://bitbucket:test@my-bitbucket',
                                            refmap: [:heads, :tags, '+refs/pull-requests/*/to:refs/merge-requests/*/head'],
@@ -34,6 +31,18 @@ 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)
+
+      expect { subject.execute }.to raise_error(Gitlab::Shell::Error)
+    end
+
+    it 'raises an unhandled exception in the fetch' do
+      expect(project.repository).to receive(:fetch_as_mirror).and_raise(RuntimeError)
+
+      expect { subject.execute }.to raise_error(RuntimeError)
+    end
   end
 
   describe '#import_pull_requests' do