Project export fails when a relation batch export fails

It was discovered in https://gitlab.com/gitlab-com/dev-sub-department/section-dev-request-for-help/-/issues/311 that whenever a relation export of a batch fails (e.g. when db statement timeout occurs) - the whole file-based export fails.

Instead of failing the export, rescue and log batch export failure and continue with the export. This way the export completes without the problematic batch, instead of failing the whole export.