cubicweb #3553549 ldapfeed: clarify errors associated to anonymous OpenLDAP bindings [validation pending]

By default OpenLDAP servers allow anonymous bindings but hide some attributes, namely "userPassword".

Therefore a missing "userPassword" attribute from the CubicWeb perspective might be caused by:

* the "userPassword" attribute actually missing from the LDAP server - but that should seldom happen,

* bogus mappings from LDAP to (CWUser, CWGroup),

* insufficient permissions to access "userPassword" in teh current LDAP binding.

Try to document that in the book and the error message.

done in3.19.0
load left0.000
closed by#cbc46f94081d [ldapparser, book] document additional error causes
patch[ldapparser, book] document additional error causes [applied]