The importer would display a 500 error page if you attempted to import
using a non-existent DNS entry. This commit rescues known network issues
and consolidates them into
BitbucketServer::Connection::ConnectionError`. The previous error
handling in the paginator doesn't work because it returns a lazy
collection.
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56154
To prevent delays in loading the page and reduce memory usage, limit the
number of projects shown at 25 per page.
Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/50021