source: Cache first mirrorlist failure error
source: Cache first mirrorlist failure error

In the case of `baseurl=`, right now librepo internally treats
it as a mirrorlist and just says `All mirrors failed` for any
error as its `GError`.

The way to get additional info is to set up a mirrorlist callback.
Command line clients are typically then expected to `printf()` it.

That doesn't quite work for us because we're a library - so one
option would be to proxy the mirrorlist failure CB to clients.  I
would like to investigate that in the future.

But in the meantime, in order to allow libhif consumers to actually
debug failures, let's just append the first *real* error to the
`GError`.

cgwalters/libhif,hughsie/libhif,edynox/libdnf,Conan-Kudo/libhif,cgwalters/libhif,edynox/libdnf,hughsie/libhif,rpm-software-management/libdnf,edynox/libdnf,Conan-Kudo/libhif,rpm-software-management/libhif,rpm-software-management/libhif,rpm-software-management/libdnf,hughsie/libhif,Conan-Kudo/libhif,rpm-software-management/libdnf,rpm-software-management/libhif,edynox/libdnf,cgwalters/libhif,rpm-software-management/libhif,cgwalters/libhif,hughsie/libhif,juhp/libhif,rpm-software-management/libdnf,Conan-Kudo/libhif,juhp/libhif