error: Use error_reportf_err() where appropriate

Replace

    error_report("...: %s", ..., error_get_pretty(err));

by

    error_reportf_err(err, "...: ", ...);

One of the replaced messages lacked a colon.  Add it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200505101908.6207-6-armbru@redhat.com>