Improve LDAP KDB initialization error messages

In krb5_ldap_initialize, don't just blat the LDAP error into the
extended message; give an indication of which LDAP operation we were
trying to do and show what parameters we gave to it.

(Also, krb5_set_error_message can handle a null context argument, so
don't bother to check before calling.)

ticket: 7739 (new)
target_version: 1.12
tags: pullup