shortnames: resolution error: point to manpage if config is absent
shortnames: resolution error: point to manpage if config is absent

When a short name could not be resolved via `shortnames.Resolve(...)`
the error message attempts to help the users solve the issue by pointing
to short-name aliases and the registries.conf.

Until now, the error implied the existence of a registries.conf which
may not always be the case.  Hence, check whether a registries.conf
was used during short-name resolution and point to the manpage none
was present.

Signed-off-by: Valentin Rothberg <c8d7c67387706f1534261b1aba83f5c355c00ff5@redhat.com>

containers/image,containers/image