Cleanup of failed download may misbehave

[wonder-sk]

Sometimes if a download fails, the subsequent cleanup of the project directory may fail as well: MerginMaps/qgis-plugin#442 (comment)
While it looks like some other app is using the downloaded files, it is possible that we are doing the cleanup too early - e.g. in download_project_is_running() we do the cleanup while some other futures may be still active: we should wait until all jobs are done before doing any cleanup.