[krbdev.mit.edu #1165] annoying error message from krb5_mk_priv()

[zacheiss@MIT.EDU via RT]

	If one calls krb5_mk_priv() without having previously populated
your auth_context with your local address via krb5_auth_con_genaddrs(),
you get the error message "ASN.1 structure is missing a required field".
While strictly true, this isn't very useful for determining what the
problem it; the error message should better indicate the problem.

Garry