API: PackageDownloader::download() method does not indicate all download errors

[m-blaha]

The PackageDownloader::download() method throws an exception only in the case of "irrecoverable" download errors. When the fail_fast flag is set to false, the method does not throw an error even if some targets fail to download. Errors related to individual downloads are reported exclusively via the corresponding librepo targets. Since these librepo targets are stored as local variables within the method, the caller of download() cannot check the status of individual downloads.

This behavior impacts reposync, which is expected to:

• Avoid failing on the first error, downloading as many packages as possible (hence, fail_fast is set to false).
• Return an exit code of 1 if some packages fail to download (now the API does not provide way how to check it)