RepoReader: disclose ConfigError

_get_repos raises exceptions with valuable info, but iter drops that info, and the user is left with no clue, eventually leading to errors like this:

Warning: failed loading ‘/etc/yum.repos.d/bindmount.repo’, skipping. Error: There are no enabled repositories in “/etc/yum.repos.d”, “/etc/yum/repos.d”, “/etc/distro.repos.d”.

This change propagates the information to the user, e.g.:

Warning: failed loading ‘/etc/yum.repos.d/bindmount.repo’, skipping: Parsing file “/etc/yum.repos.d/bindmount.repo” failed: Parsing file ‘/etc/yum.repos.d/bindmount.repo’ failed: IniParser: Missing section header at line 1.