Add LDAP error message explanation.

This is a change that should make clear that besides error messages thrown by OMERO, there is a set of Spring- and Java-specific messages that have to be looked up in external sources (i.e. Spring API docs and Java API docs). See https://www.openmicroscopy.org/community/viewtopic.php?f=4&t=7426#p14069.
"or by Java"? "documentation" in full?