samba-tool: Rework ldap attribute fetch in classicupgrade for missing attributes

samba-tool: Rework ldap attribute fetch in classicupgrade for missing attributes

Is is not required that these additional attributes be filled in, so
catch KeyError in both the nsswitch and ldap backend case.

We rework get_posix_attr_from_ldap_backend() so it raises KeyError
rather than trying to return None, and does not ignore other errors.

Andrew Bartlett

Tested-by: Chirana Gheorghita Eugeniu Theodor <office@adaptcom.ro>
Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Jelmer Vernooij <jelmer@samba.org>