Better recovery from key download error

[DrDaveD]

If a key download fails after a directory in /etc/cvmfs/keys was created, remove the directory and don't try to create the corresponding repository.

[DrDaveD]

Actually this may have only been caused by a bug that has since been fixed, and so may not be necessary.

[DrDaveD]

This is still a problem when a key download fails. It leaves an empty directory and the next time it sees the directory and so doesn't try again to download.

[DrDaveD]

@cvuosalo I intended to ask you to fix this in the previous release but forgot about it. It's not urgent, but please put it on your list to do eventually.